Ugh, the anti-aliasing kills most of them for me. I realize I'm probably in the minority, and my opinion might change as screen resolutions get better and my eyes get worse, but I vastly prefer crisp fonts for screen use.

When I switched from Windows 2000 to Ubuntu 6, I spent hours tweaking fonts.conf in the hope of getting something that looked as good as the standard Windows fonts, without much success. It's gotten easier now.