wayned

From what I've read, creatine can have a positive impact on muscle mass by about 5%. But of course that varies greatly from person to person, being influenced by diet, workout routine, age, genetics, etc. Its considered relatively safe, though there is some concern about kidney function impact. Overall, there are very few thorough, peer reviewed, long term studies, so its really mostly anecdotal.
Ultimately, having a solid, protein rich diet and lifting weights consistently is what is going to make the biggest difference. But if you're looking to supplement, talk to your doctor/dietitian before starting anything.