Gave some trials a go, wasn't greatly impressed. For me the hype of this service was being able to play any game, at highest settings, but as far as I can tell they're pretty low settings - is this the same with full games? From what I've seen I can play better looking games on my average 260 setup, without the latency. Any latency is also quite annoying playing FPS mainly I found, as the reactions just don't feel right, and I'd hate to think I was compensating for lag as I got used to it.
Overall I guess it's great for handhelds/basic laptops while on the go, and for quickly playing demos of games you're thinking of buying.