I had a chance to work with the Blueprint CSS Framework the other day, and I was very impressed. At first I found the class names a bit weird and it bothered me a little to use them (span-10, push-2), but after using it to build a template, I found that I no longer cared about the names. In fact, I ended up liking the names as they accurately reflected their purpose. I must say, making the template was so much faster and easier using the framework that I really wish I had known about the framework long ago. How many wasted hours could have been spent doing what I really love, php coding.
The features I liked most are its cross browser unification (reset), print & typography separation, and overall the ease at which I could expand it to suit my needs.
The only thing I think it could really use is an expansion of the form section to have some included icons for error, success, etc. and a few more like info, alert, help, etc and some default form error styling. That would mean less work overall setting up shop quickly. If I wanted different icons or colors or etc, all I’d have to do is override them in my own style, though if done generically enough why bother at all.
Another really useful tool was Boks, it made layout creation a snap, and also speed up learning the usage of the framework.