All the relevant facts have already been mentioned. Besides that, I can only tell from my own experience that I was a bit disappointed by it's effect. Especially because I was told it might have a bigger effect on me, as I'm not too fond of red meat, which is a natural source of creatine.
I must say I had the feeling that the first few workouts after taking creatine for the first time, I really had the idea that I could lift more. But after like five times, this feeling was totally gone. My progress wasn't going any faster than before and I didn't gain weight faster than I used to.
After like a month, I got the feeling that progress was actually speeding up a little bit, but this may just as well be because of the fact I just trained more often and stuck to my diet more.
Then again, I'm kind of a beginner in all this. It's probably for good reason it's such a popular supplement. As said by the others above, perhaps it's best to try it out for yourself. I'm finishing my can and after that, I'll keep a close eye on my progress without, so I can compare to say for sure whether or not it did anything for me at all.
Finally - and I don't know the other's take on this - I believe tests have shown that taking extra doses of creatine for a longer period of time may compromise your body's ability to generate creatine by itself. Unless anyone can prove me wrong, I'd advise anyone to have a break of taking creatine supplements after like two months or so.