At high resolution such as printed text at small sizes people tend to find seriffed fonts quicker to read. Something to do with the serifs leading the eye. At larger sizes they just add distraction like any other decoration, and at low resolutions on small-ish text they can produce quite a mess legibility wise unless the font is very well designed with low resolution rendering in mind. Sub-pixel rendering help a little, but not nearly as much as some seem to think.
Some people insist on seriffed fonts even for small text in low resolution environments because they find them easier to read elsewhere, without considering that the different environments (screen and print) have quite different properties (or being blind to where the difference is, I know someone who blames the OS's font renderer when he is asking it for the impossible). Of course as screens progressively move towards higher PPIs (I'd love a 24"+ display with the PPI of my phone's screen) the low resolution rendering thing will diminish as an issue.
Some people find it easier to process, but I never found out why.