If you can see cracking on the inner side of the serpentine belt, I'd replace it. The cracking will soon be followed by small missing chunks of rubber.
Yes, it's not a difficult DIY job. The biggest problem is getting access to the tensioner to loosen the belt. I was able to squeeze a standard ratchet in there on my car. There are also tools like this:
http://www.autobarn.net/lis57600.html 
