I don't usually start off with a framework because then I'm dependent on someone else for it. And if I have a problem, I can't fix it, I have to wait for someone else to.
Yes yes, I know if I find a bug in bootstrap, I can fix it myself or report it, but if I have to learn the inner-workings of a framework, it's not really a framework I want to use, is it?
Having said that, my boss asked me to re-write the front end of an enterprise class application from Flex to HTML5 (in 2 months😱) and I'm using bootstrap and it's going very fast.