Corywrote a review of Suunto Ambit2 Sapphire Heart Rate Monitor on July 15, 2013
Familiarity: I've used it several times
I bought this to replace my first gen Ambit. The Fused-Alti feature and more memory and battery life are what made that decision. The Fused-Alti feature incorporates both barometric and gps data to give a more precise altitude reference, though I've found that measurements can vary a bit depending on the terrain you are on. For example, on a small summit pinnacle like Lone Peak here in UT, which has a 10X10 summit area surrounded by cliffs, the altitude reference is off by a couple hundred feet if the gps fix in the watch wanders off at all. However on a broad, rounded peak, the altitude is usually within 10 feet of accurate. Pretty cool feature really, and I've found that I get the best results if I fire up the fused-alti about 20 minutes before getting to the trail head.
Another feature that is pretty cool on the watch is the auto-pause feature, which a lot of the Garmins have. I've found it works well for faster moving activities like biking and running, though while scrambling slowly up some exposed ridge it can be a nuisance.
The heart rate strap that comes with this... sucks. I found that communication between the strap and the watch would continuously loose sync. I ended up purchasing the Garmin ANT+ strap sold here on Backcountry which fixed that problem. I suggest getting the $600.00 version of this watch and then getting a different strap altogether.
Overall the build quality of this watch is pretty damn tough. That stainless bezel and the mineral lens haven't shown any signs of wear at all over the past couple months I've had it. The casing is the fiber-resin, which looks to be the same housing material of the original Ambit. Overall the watch is pretty decent, sans the issue with the heartrate strap provided by Suunto. This watch is tough enough to take a good beating out doors, but classy enough to be worn formally as well.