I have been seeing a lot of guys in my gym using creatine serum (the type where you put a few drops under your tongue) instead of the powder. They say it works better. Is there any advantages or disadvantages of the serum over the powder?

I believe the serum is a marketing scam. I don't know about anyone who has seen any results by taking it.

I suggest you read this article http://www.ast-ss.com/articles/article.asp?AID=88

creatine left in solution converts to a substance useless to the body.
possibly the largest scam in the industry

johnd