Never used a framework so far really. They provide a good structure, but sometimes I feel that if you don't use 100% of the functions/options that it's a bit of an overkill.
I've usually just use a normalize/reset CSS and made my own little framework when needed and that works fine so far.
I can imagine that if you need to whip up a site really fast, that then it's very useful to have a framework to hit the ground running, but for me personally, I stick to my own stuff. ;)