I tried it a few years ago -- a couple of 8 week cycles (8 on, 2 off, repeat). It is supposed to help increase lean muscle mass, improve performance in high-intensity exercise, increase energy levels, and speed up recovery rates. It is supposed to super-hydrate the muscle cells with water. Those are the claims.
I liked it because I could REALLY tell that my muscles were more pumped (in my upper body) and I DID feel stronger in all my lifts. I guess that is the feel of the "super-hydrated" muscle cells. But it made me look smooth due to holding onto all the water and I put on about 8 pounds (which, I know was mostly water). I just felt too "puffy"...if that makes any sense.
At that time in my life I wasn't very good at keeping track of information about my workouts so I can't say for sure if I gained any muscle mass at all. But if I was lifting heavier and recovering faster, it is reasonable to think that I was gaining some mass.
I may try it again in the future -- but not in the summer. I don't need any extra water hanging out in my body while trying to wear a swim suit ;-)
Good luck,
Shonie