I rented Warhammer 40k: Space Marine for five days. I beat it in two extended sittings, and I still have four days of play left. Too bad the OnLive version doesn't support multiplayer, or else I'd probably have a reason to keep playing it after I beat single player. Really not enough content to warrant $60.
On the other hand, I got Half-Life 1 and Star Wars: Dark Forces II on Steam through holiday training shenanigans, and so far those are pretty good. Old-school shooters are still some of the best in my opinion, you got a lot more bang for your buck.