Same here really - for a site that I know wont need to have much new content added much I just go for straight XHTML and CSS. Benefits are you are taking processing load off of the server, and its more secure (i.e. less chance of exploits in your code).
Ones where I know I will be adding content and want to be able to search and ahve autoamtic site maps made etc, CMS is the way to go.
Before my latest site, I never programmed my own stuff. Used Mambo and phpBB instead.
But there wasn't anything available for my next site idea so I just built it myself. Took 6 months, but I think it was worth it. I'm considering writing my own CMS for my other sites too.
One thing to be aware of with rolling your own code is security.
I did my own CMS/blog type thing years ago; not long after someone had found a way to use it to setup a phishing site on my domain! :O Learnt a lot since those early days...