This is something I really enjoy. Had some great chats when people noticed I was playing a game they liked.I try my best to at least chat a little to people I add, or to those that add me. Just a comment here or there on a game they are playing, or if they are new to Steam just some pointers here and there.
That said, I never dared to initiate a chat myself with someone I hadn't talk with before (even though I was tempted when I saw somoene booting up F.E.A.R. a couple of days ago), since I feel I'd be bothering them when they just want to play instead of talking to some random fool.