I use ++ and -- almost like bookmarks. If I think something that's been written is worth referring back to, I give it a ++. Also, if the person responding has, in my opinion, taken some time to write something worth reading, I'll click the lil' ++ button.

If it's rubbish, flames or a 'do my homework for me' question, then I give it a --.

The latter I do much less frequently than the former.