Aside from the fact that you don't have to pay for any software on a linux system? I run OSX for the things I HAVE to, linux for the things I WANT to.

I run linux on a 4,1 series aluminum macbook pro. Runs really well, though the new kernal is killing my battery life for some reason. I've gone through a lot of distros on this laptop, so far Fedora 15 seems to run best! Mint, Sabayon, Debian, CentOS, and openSUSE were all pretty nice too.