Creatine is excellent stuff, it totally lets you get a few more reps out of each of your routines and by doing that, lets you gain better.

It has some downsides, I'll list them.

1) It can cause you to retain water.
2) It can cause Liver problems for those who already have weak Livers.
3) It can cause semi-severe stomach aches.

I take 5g of Creatine every day (they actually want you to load up for a 2 week period with 20g's a day, and then just take 5g each day, before you workout, after the loading period -- but I feel uncomfortable with this amount). Even with just taking 5g each day (with Grape Juice, or any non citrus juice), taken about 20 minutes before I work out I notice a significant difference in how many reps I can get out before I "fail".