I've got one - I wear it when kayaking, open-ocean rowing, and other boating activites. I also wear it when flying seaplanes and such around here.
I came very close to deploying it last year, when I capsized in the 45 degree water, and was unable to right my vessel due to combined equipment/skill failures. Fortunately, my "waterproof" ICOM marine VHF transceiver lasted long enough to get off one Mayday before it shorted out. (Lesson learned: always, always, always test your gear under operational conditions!)
The EPIRB function is handy, but a bit painful to use - you must unscrew and lay out two wire antennas, and the orientation is important. This can be tricky while you are floating in the cold, cold, choppy water...
As a watch, it isn't that great - the user interface is miserable, and it's kinda clunky. Light, though, with the titanium case and bracelet.
Its chief virtue is that it is physically attached to your body. Had I had one of the new, small manually-operated personal EPIRBS when I capsized, I would have had it neatly stowed on the boat, and had some trouble getting it into action.
I'm going to get one of the newer, even smaller personal EPIRBS this year, though, and keep it attached to my vest - the ease of use is much better, not having to deal with the little wire antennas. Plus, if you deploy the watch, it has to go back to the factory for service, which can be a bit of a bother.