I checked the Onlive site earlier today to see if the service had been released in the UK (last time I checked was like November 2010), and it looked like it had. It turns out that they are just accepting sign-ups from anywhere right now, with the drawback of only being able to use US data centres.
So the lowest ping time you can get from the UK realistically is about 100ms. This especially impacts Onlive because it is streaming a constant video and audio feed back - the normal lag compensation methods used in games today aren't going to cut it. Having a low ping with the service is crucial.
Because of this it wasn't as responsive as I'd have liked, but the games were playable.
You can play repeatable 30 minute demos of loads of games on the store, so you can start using the service for free to try it out without having to enter payment details.
Below is a demo of Batman: Arkham Asylum to show how smooth the service is (framerate wise, not delay wise) and to showcase the service's UI. It isn't a fair representation of Onlive's video quality as it has been compressed, but the screenshots below it do.
I was able to load a game up in under 2 minutes and start playing, this without downloading any more than the Onlive installer. Below are some screenshots taken in-game from my computer (bottom is Assassin's Creed: Brotherhood):
It's nothing fantastic, but 720p is by far enough for most.