Having spent a 20+ months working with Thesis and Revolution themes, I've been tearing them apart and not feeling like I could actually build my own.
I'm not really enthralled at the prospect - nor do I really have the time - but there are certain advantages to building my own - like optimizing the code to be light and effective SEOwise. And there are certain advantages to buying one or hacking one - less time to implementation.
What's your preference?