I use LoveFilm here in the UK which recently got bought out by Amazon where you can rent DVD's, Blu-Rays and games. You can also stream to the PS3 and on their website but they charge extra for good films. You can get some older comedy films like Wedding Singer and Hot Fuzz streamed and action ones like Lord of War but that's about it. The PS3 service is brand new in the last few weeks so they're working on what's available.
I don't really use it as I should, I pay around £10 a month whereby I can rent 1 disk at a time for as long as I want. I've had Windtalkers on rent since August so needless to say, I could have just bought the blu-ray a few times over by now. The post is also getting worse but that could be either LoveFilm or the mail. I used to have a 2 disk at a time package but I wasn't using it and got to the point where I cancelled my account for a year. I'm on the verge of that now because I just don't have time to watch a film now with work, I'm just using it to see every episode of CSI:NY from the start.
I've heard rumours online that Netflix are due to invade the UK market soon so I'm hopeful that they will provide a better streaming service that may even find it's way onto the xbox.