Does anyone use it? I've only ever tried it for trials but it seems like a pretty cool service and rentals are available alongside permanent purchases. Having a massive game running within minutes is ace, although the graphics seem to take a hit so be warned if you're bothered about that sort of thing.
The best feature? Even games that don't get a Mac release are on there and the majority of the publishers let you play them (there are some exceptions I've heard).
Finally, if you're UK based and you've never used it before, sign up this week and you can get a game for
£1. There's an impressive selection included within the deal so it's well worth a look.
OnLive
EDIT: I should also mention that system requirements are very low which is pretty useful if you're running an older machine. Just watch your data usage, if you're capped this seems like it could run up your bill!