Suunto Lumi was my second choise and just fifteen minutes after buying the Core I regretted not buying Lumi instead. Core is HUGE, basicly a UFO strapped to your wrist and distinguishable from at least lower Earth orbits. Lumi is essentially the same device but it's half the size, so it's more of the size of a conventional men's watch. For some strange reason, Lumi is marketed for women. While the Suunto Core marketing talks about traveling in wilderness and scaling mountains, Lumi marketing talks about finding directions in rock festivals. Ok, the wrist strap is a bit ornamental in a feminine way and a regular black strap might be more mannish, but Suunto sells separate straps so that should not be a problem. The only missing features in Lumi are the rotating bevel (it's useful) and the depth meter (very rarely useful). And Lumi is 50 euros more expensive than Core (250 vs 200 euros).
In this article, I will go through the main features.
Construction
As I said, it's HUGE. The only reason that I can think of for the size is that you can use it with the gloves on. Strangely, the Suunto webpages do not advertise this reason. The size is a problem in so many ways. It doesn't really fit under the sleeves and it makes more difficult to don a jacket. As I need the watch when traveling, I don't want my watch to draw the attention of the natives who make 30 euros a month and the watch worth their six months salary (not that my pro-looking camera equipment draw any attention). On the other hand, I wish I had had the watch when I went snorkling in the coral reefs at Keys. Anyhow, I would have preferred Suunto Lumi.
Despite the size the watch is really light. The size is apparently just empty space and not because the electronics require that much space. It's not light enough to float though - I tried. The wrist strap is a bit gelatine-like material and it's somewhat difficult to open the lock. You can not adjust the strap, or more accurately, you have to adjust it every time you put it on your wrist. There are so many holes that it's not easy to know which one is the best. Because of the gelatinish material, the skin under the strap sweats like a pig. The reason to have a metal wrist strap is not that it looks pretty but because it conducts heat.
Another problem with the strap is that it's positioned so that it is not possible to put the watch on a table surface (see the figure below). This makes use and especially calibration of the compass very very very difficult, because the compass must be leveled very precisely. Also, the back-side of the watch is nowhere near flat. It also makes setting compass direction on a map much more difficult. Have the designers actually used the watch?
Some people at Suunto discussion forums complained about too loose or tight compass bezels. The rotation stiffness of my bezel is excellent, but in small scale the bezel is loose and moves even if you touch is only lightly. The bezel has 60 steps so for compass purposes the steps are 6 degrees. Unfortunately, the compass scale is not written in the bezel and it only has letters indicating the cardinal directions. Some other models have a scale.
The glass is mineral glass and it is a good thing that it is not plastic as most cell phone displays are. Plastic displays scratch really easily and they are full of scratches in a year or two. The most scratch-resistant glasses are the sapphire-coated ones, but ordinary mineral glass is usually enough. The surface is convex and looks good. If you end up stranded in an island, there's no point in removing the glass and using it as a burning lens to light your campfire, as you can just use the reflection for the same purpose (if it only was reflectively coated). Oh well.
The Clock
The clock is such a basic functionality that it is hard to screw it very badly. I have not used the watch long enough to say how well it keeps time. At first, I thought that the watch lost time very fast (10 seconds per day), only to notice that it was my computer's clock that lost time. When comparing to accurate time, the Core seems to have gained about 3 seconds in 4 days, which is not good, but I'm again not certain that I set the time correctly in the first place.
The clock has a number of views: just time (hh:mm), time and date, time (hh:mm) plus seconds, time (hh:mm) plus time at a secondary timezone, time plus sunset/sunrise times, timer, countdown timer. Why are there so many views with just minor differences? What's the use of showing just time and not date or seconds? Why isn't there a view that shows all time (hh:mm:ss) and date in one view?
The only really annoying thing is the countdown timer. I need such a timer almost daily for foodmaking purposes. Setting the countdown time requires that you gough the menu: click Menu (2s) - select time-date (2 clicks) - select countdown (2 clicks). Then you can set the countdown time. Why can't you just set the countdown time in the watch view?
The Altitude Meter / Barometer
The altimeter in Suunto Core is supposed to have resolution of one meter. I haven't been able to measure this very accurately yet, but it seems believable. I used the altimeter in one measurement to measure the relative height of two pillars 400 meters apart and it gave constant readings within some 2 meters. The problems are in calibration. Core allows you to calibrate the altimeter in two ways: by a known altitude or by local atmospheric pressure at sea level, which you can get from weather stations.
The problem with the calibration by the atmospheric pressure is that Suunto Core allows setting the pressure with the resolution of one hPa, but the difference of one hPa is 27 meters in altitude! Weather stations give the pressure with one decimal. The advertised one meter altitude accuracy means that Core should be able to measure pressure with about 0,03 hPa resolution, so why can't I calibrate it even with one decimal?
Another problem with the altimeter and barometer mode is that it's a lot of work to switch between them. Switching between them through the menu (which requires 2s press to open) takes 10 button clicks in all. Not very easy! There is an "automatic mode" though, which should have the altitude meter on when the pressure changes fast (i.e. when you're moving vertically), and barometer on when you're stationary, but it takes a few (2-3) minutes to switch to the altitude mode and a lot longer (15-30 minutes) to switch back to the barometer mode.
The altiude/barometer mode also shows the temperature. Unfortunately, the body temperature affects the reading strongly, so when you're wearing the watch in your (naked) wrist, it will show 30-33°C if real temperature is 20-30°C. I did a small experiment: digital thermometer shows 28,9°C and Core shows 30°C. I put the watch in my wrist and wait 5 minutes. It will show 32°C. So, it's basicly useless when you're wearing it. Taking it off doesn't help very quickly and it takes maybe 15-30 minutes to stabilize. I would really have expected that the watch would compensate for the body temperature at least a bit, possibly by measuring the temperatures from back, middle and front sides of the watch and extrapolating the temperature. Or just by using a thermometer that measures the air temperature with infrared or something. I suppose there might not be a perfect solution, but currently it's just bad.
The Compass
The compass has been nearly useless so far, because it's highly sensitive about tilting and loses calibration all the time. So, while the specs say that it should be accurate to 1 degree, I would say that it's accurate to maybe 180 degrees, because you never know when it has lost the calibration completely or just partially. It doesn't need to be accurate to one degree and some 5° would be acceptable, but now the accuracy is much worse.
The compass is really the most important feature of the watch for me. I need it for urban orienteering when I'm traveling and for rough directions for amateur astronomical purposes. For example, I want to know the rough direction of the Sun or the Moon or some planet or a star that I see in the horizon. When I'm gazing for satellites and there's a predicted satellite flash at a certain time, I need to know the exact direction where it will show. If I see a meteor (or even a fireball), I need to be able to estimate its radiant. At night, I can usually see the directions from the stars very easily, but in daytime or in dusk I really can't. Finding Venus or Mercury from the sky during daytime requires knowing the exact direction.
Occasionally, I get happy when the reading looks reasonable, but then I realize that it's the same as a dead wall clock being correct twice a day. Typically, the error is about 20-50 degrees, so in a real situation, you would not really know that there's something badly wrong. For note, I have calibrated the declination, and it's not huge anyhow (+6.5°), so it's not that. Reading the direction at one place can give steady readings, but fifteen minutes later it gives totally different readings. Sometimes the compass goes to an oversensitive state in which, if I turn 90 degrees, the reading changes 180 degrees.
The problem may be that the compass loses its calibration very easily and needs to be recalibrated, in practice every time you use it. The watch doesn't actually allow recalibrating the compass the same way as it was calibrated on the first use. The manual says that you can recalibrate it just by rotating it slowly around for a full circle, but this method is a bit uncertain as the watch doesn't actually indicate when it has been recalibrated. The watch needs to be kept exactly leveled during calibration, but the badly shaped wrist strap (see above) makes it impossible to level the watch on a flat surface, so you need for example a bottle to put it on. But you can't carry a table and a bottle around the world in your backback, can you?
The User's Manual says that the compass must be held leveled with the ground for it to be accurate. That' s an accurate statement, even if the compass is not. Tilting the watch even slightly can change the compass reading radically, so that a 1° tilt can sometimes cause 2-3° error in the reading. Sometimes, possibly if calibrated better, the compass is less sensitive to tilting and behaves even reasonably well. The manual says that the watch would warn about tilting (Finnish manual says - English doesn't), but it doesn't give any warning for me however much I tilt or not tilt the watch. I've experimented a lot with this.
The sensitivity to tilting creates a practical problem: leveling is not easy. You can level the watch most accurately by looking at it horizontally against the horizont, but then you can't read the compass reading without lowering it. The watch isn't really designed to be held leveled. The wrist strap prevents from actually putting the watch on table. I've found a cardboard tube from an empty toilet paper roll to be useful for leveling the watch.
Pressing the light button creates a really weird light show: the light goes off and on about two times a second so it's flashes quite annoyingly. The display font in the compass mode is really ugly.
Other
A hint for Finnish buyers: Kello- ja kultaliike Suominen in Turku sells these for 200 euros while other places demand 250. Also the price for Lumi is 50 euros cheaper than elsewhere.