Personally, excluding maltodextrin and protein (which are technically only food), creatine is the best single supplement I've used. After 4-5 days of loading 20g in divided doses, I experience a 10-15% improvement in strength, recover much more quickly after intense expercise, and have more "finishing power." By finishing power I mean, I'm able to complete the last rep more frequently. It feels as if a "reserve" is kicking in. I know, it sounds like bullshit, but that's what it feels like. Without creatine, when I fail, that's that - I'm done. With creatine, it feels like I'm about to fail but I'm able to squeeze out the last rep instead of the spotter needing to pull it off of me.
Creatine also gives my muscle a more full look. Some say that it bloats, but I've never experienced this. However, It WILL cramp, your calves especially, if you don't drink enough water.
Creatine doesn't work for everyone, but it works well for me.