Personally, I found when Slashdot went over to sans-serif fonts I read a lot less of the comments than when they had a serif font. My eyes just seemed to get tired more quickly.
Having done some fiddling around with fonts after noticing this effect I've found that I prefer serif fonts for large / long lumps of text. However, I'm sure if you ask around you'll find lots of people who prefer it the other way around. I've been thinking of adding a little control panel to some of my sites to let people easily switch between the two. It's probably not worth it for short pieces, but for articles it could be worthwhile.
Having done some fiddling around with fonts after noticing this effect I've found that I prefer serif fonts for large / long lumps of text. However, I'm sure if you ask around you'll find lots of people who prefer it the other way around. I've been thinking of adding a little control panel to some of my sites to let people easily switch between the two. It's probably not worth it for short pieces, but for articles it could be worthwhile.