Creatine is probably one of the only supplements, if not the only one, to have been studied in major universities (Harvard, Columbia, UCLA, Berkeley, Cornell, and most other universities/colleges). Overall, the data has been mixed, with some studies showing creatine to be effective in improving athletic performance, while other studies show the opposite. Either way, if I was going to buy one supplement, and one supplement only, Id buy creatine. At least it has some validity behind its claims. Better than shitty muscle-tech products.
A statement from the Mayo Clinic:
Several high-quality studies have shown an increase in muscle mass with creatine use. However, some weaker studies have reported mixed results. Overall, the available evidence suggests that creatine does increase lean body mass, strength, and total work. Future studies should include the effect of individual differences such as fitness levels, sex, and age.