I'm a heavy sleeper, and it always wakes me up if I have someplace important to be. For peace of mind, I still set a back-up alarm across the room to go off 5 minutes later. To the watch's credit, the backup has never been necessary in the many months I've used the watch. When I want to wake up on the weekend, though, I tend to fumble with the buttons when it vibrates, and turn it off without actually waking up. (This coming from a guy who used to require a password-protected alarm clock to wake up for work.)
Pros: Inexpensive, easy battery replacement, clip on back so you can use it without the strap, and totally reliable for me (depends on how you sleep and wake).
Cons: Bulky, slips off the strap when awake (but not yet while sleeping, and I toss and turn a lot!), and very short backlight time.
Other notes: Another commenter already pointed this out, but the instructions that come with the watch made no mention of the hourly chime. Hold 'Reset/Time Set' and press 'Mode' to clear the 'SU M TU W TH F SA' to deactivate the chime. There's also a snooze function, which I think is activated by hitting any of the buttons except the light button.
Now that I trust that vibrations will wake me, I'm going to give the
WobL 8 Alarm Vibrating Watch - Silver a try, mainly because it's less bulky.