Creatine is a somewhat rare supplement in that there is plenty of evidence that backs its use. It improves strength, enhances recovery, and increases muscle mass. It does not however, work on every activity. The evidence on creatine use to improve aerobic activity, high intensity exercise, sprints, and decrease body fat is at best, mixed.
There is research demonstrating that consuming 20-30 grams of creatine per day for up to a week increases muscle creatine stores, enhances recovery, and improves performance. Research also shows that small doses of creatine, as low as 2-3 grams per day, can maintain or even increase levels of muscle creatine.