agree with all those who said that it very much depends on the font - used to be a "Sans" fan, until I discovered
Gentium, and more recently
Lexia DaMa (a free
Caecilia clone), now the standard font on my reader. Both Gentium and Lexia DaMa are Serif, but there are more Sans fonts that I haven't experimented sufficiently and that look beautiful, as e.g. the
Ubuntu font family. Some fonts have both a Serif and a Sans serif version, e.g.
Droid.