I just replaced the disk in my 17" yesterday. It wasn't too bad, I thought it would be harder than it was. As long as you follow the instructions at pbfixit.com, have the appropriate tools, and take your time, it should not be a big deal. From start to finish it took me about 20 minutes.
Although, if you have never worked on the inside of a laptop, and are worried about messing something up, then you may want to take it in to have someone else do it.