How Effective is Creatine?

I'm looking to build muscle size and as of right now I'm doing pretty good. I'm doing 180 days of P90x and I'm on week 7 as of right now, I run 1.5 miles a day and lift weights by night. I'm a scrawnier guy but I put on 10-12 lbs. since the day I started. I'm looking to weigh 160-170 and I currently weigh 157.5. My friend told me about this supplement, Creatine, and I was wondering how effective it was at building muscle? I'm looking to gain some size and honestly 10-12 lbs. is more than I've ever gained in my life, I was fluctuating between 146-148 lbs. for the several years. So how effective is creatine? Should I use it?