I use boilerplate, a stripped down blueprint css:

http://code.google.com/p/css-boilerplate/

I'm not a semantics-nazi by any means, but I've used grids at different times and... meh, I'd usually rather just throw a few float:left's together and keep it clean.

I am using blueprint css in my latest website ( http://overtrainer.com ) mainly just because I am too lazy to learn css correctly ;)

i've made use of blueprint and grid.

the concept is useful but don't use them as a crutch for learning CSS