Traditionally, I'm not much of a framework guy. The big issue for me is that I don't like learning a framework, only to find out it is not going to work in the long run. Whether the framework actually doesn't work or I'm just avoiding it is arguable. One area that I've found frameworks to be extremely helpful as of late is in CSS. I used to really enjoy the idea of CSS, but now I dread it. Browsers make things hard and realistically supporting them all is a nightmare. Javascript is having something of a hey day primarily because there are a set of libraries that make the browser issues manageable. CSS frameworks make an effort to do the same thing.
Not researching very deeply, I've gotten the impression that CSS frameworks are somewhat controversial. If you don't understand how to use CSS, then don't hide it behind a framework because it will only bite you later. I'm sure this is true to some extent, but at this point in my life, I'm sick of messing with hard issues that don't matter that much. I want two or three columns and my text to line up right so I can move on. I gave the YUI Grid CSS framework a go and it made things easy, so I was done and could move on.
One negative I've heard about the YUI CSS framework is that it forces you to change your markup. This does kind of stink, but lets face it, if I have to redo the template, I'll change the markup anyway. I'm sure there are big enterprise problems and issues that have to meet some ridiculus requirements, which make this idea an issue, but I'm going to stay away those.
There are other CSS frameworks out there that I might look at sometime, but not anytime soon. I'm pretty happy with YUI. I'm also happy I found a good tool for getting rid of some of my CSS issues so I can move on.