Much has been written about CSS frameworks, should you use them or not, and which is the best. When I first investigated them my starting point was here at top 12 css frameworks , and I looked at all twelve, played around with a few, but really was not that impressed.
These frameworks seemed fine in principle, but either seemed far too heavy with large css files to download, and / or, just seemed not that user friendly.
Recently however I've come acros the 960.gs framework, and I have to say I'm impressed. Only time will tell if I stick with it.
Update: October 2010. I'm still using 960gs, and have just switched this site to using it.