I basically look at it this way: was it "invented here"? If not, then I build it myself.
But davidw seems to be thinking this way: http://www.joelonsoftware.com/articles/fog0000000007.html
It might be fun to build you own CMS but, for many of us, could be a distraction from what we actually want to make.
Yes, my answer was a bit tongue-in-cheek. It's almost always better to see if there's some open source code you can build on.