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.
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!
I actually made a point to try it out. I really like the idea of a streaming video game service, moving the high-end system requirements away from your desktop and just relying on a very strong connection. I do believe this is the future of gaming.
I'm about two-thirds of the way through Batman: Arkham Asylum. I tried to pick a game which had some well reviewed graphics (to check the quality of the stream) and which relies on somewhat quick controls (to check the lag issue).
My overall experience is good, even though there are some moments of frustration (when your connection blips or Batman suddenly sprints for the wall because of a connection problem). But these are extremely rare, only happened two or three times.
Surprisingly the graphics are very well streamed. There are a lot of super fine details that come through looking just like a 1080p YouTube video, and on my three Apple LED cinema displays the graphics look very impressive. Occasionally an artifact appears, and very rarely you have some trouble seeing a super far distance (because of very fine details), but OnLive's streaming technology makes these only momentary problems and quickly renders everything properly. If you stand still for an instant you can catch the rendered making the details sharper.
The game demos and instant-on functionality of these games are really absolutely awesome. I also like how I can switch between machines (my desktop, laptop, and soon my iOS device) and pick up in any game where I left off.
So, my review; if you've got a good enough internet connection and don't care about perfect graphics and just like to game, try out OnLive. It might not be for everyone, but I've been nothing but impressed. After I finish Batman, I will certainly be looking at their other offerings alongside looking at Steam for my next purchase.