Creatine is probably one of the most widely studied supplements out there. While there's concern with kidney damage (that study was done on patients with renal failure, go figure) and muscle strains (more than likely secondary to dehydration), it's by far one of the safest supplements to take, and most effective.
If your are going to take it, make sure your taking in a LOT of water a day. Think 4 litres.
"You do not truly know someone until you fight them."
- Seraph, The Matrix Reloaded