Wow...this one keeps coming up.
I've taken it with great success. You certainly can't know for sure if it's helping you because you don't have an identical control of yourself along for the trip, Although, I've experienced altitude sickness regularly, and at even moderately low altitudes (7-9k ft), and was able to ascend from sea level to nearly 11000 feet in one day with not so much as a headache. I did have tingly hands, peed a bit more often. Also, soft drinks and beer tasted flat for a few days.
There are a lot of factors that contribute to AMS--hydration, rest, fitness, exertion, underlying illness--but Diamox can help you acclimate, and has been shown to reduce AMS in double-blind control studies.
It's important to say that if you have any symptoms of HAPE or HACE, Diamox won't do anything to cure that, and you should descend immediately and get medical attention.
Poke around on google and you can find a lot of information about it (percentage of people who suffer side effects, its effectiveness, etc.).