I find some of them amusing, in a that's-so-wrong kind of way, but think only a real jackass would wear some of those in public. There are offensive ones and some that are just dumb, but I really like the band geek hero shirts. Buying my cousin the Accordion Hero shirt, 'cause his parents used to make him take accordion lessons as a child and, even though he plays bass in a rock band now, he's geeky enough to find it funny.
I also like the Voted "Most Likely to Travel Back in Time" Class of 2057, but I'm kind of a sci-fi geek, so .... *shrug*