Maybe a telling fact: I've never used one for a real project.

When I'm the CSS lead on a project (kinda my thing), I feel most comfortable when I'm intimately familiar with everything happening in the stylesheets. I get that intimacy by writing it (and reviewing what other people write). Not to mention I enjoy writing CSS.

I get squirmy when using any outside CSS at all.

But as usual, I'm of two minds. I'm also extremely impressed by some CSS frameworks. Bootstrap is kind of amazing and I feel like the "ugh, bootstrap is everywhere blah blah" sentiment is overblown and unnecessary. I'm super impressed by Foundation. I think it's approach is probably my favorite (I get good results applying it to raw semantic markup.) It's starting to get a little weird though recently with all the different varieties native apps and stuff.

For the teams out there that don't really have a CSS lead, or don't have one that is excited about that being their job, you can likely get a lot of productivity out of a framework.

There are also about 20 billion of them because of this weird thing that happens where people that have some success writing CSS from scratch want to turn their thinking into a framework for the rest of the world.