I'm a big advocate of it.
I've used it in the past and am in the midst of getting back on again. 21 is correct in saying, though, that you must work out regularly in order to see the full benefit.
In roughly a two-month cycle, I gain between 10-12 pounds, tighten up considerably and also experience big jumps in strength. The last time I used it, after about seven weeks I had upped my (unassisted) bench to 295 pounds, and did as many as 15 reps at 225. I work out pretty regularly even when I'm off it and without it, the best I've done on the bench is 255. Really, the difference is remarkable.
Now, the down side. No one really knows the long-term effects of regular use. I also am fairly certain that I separated my left shoulder from lifting such heavy weights on a regular basis -- the tendons and ligaments just aren't used to such big increases in such a short period of time. I'm still feeling the effects of it more than a year later, too.
But it all depends on what you're looking for.
There's different ways to take it, also. You can buy pure and mix it with a delivery powder, or buy it already mixed.
Anyway, feel free to PM me if you have any more questions or want me to get more specific.