YUI css is designed to do a few layouts well and make other layouts possible. Its not bad, but it is clunky to work with and a pain if you don't fit one of its proportional models precisely.

Blueprint is more logical and conceptually easier to understand - everything is put on a flexible grid and you choose how many grid units you want to use for each layout block.

I dig the rest of the YUI toolkit, but compared to blueprint, YUI CSS feels like a hack.