Ok, played through the first couple areas of Prince of Persia: The Forgotten Sands. The controls are a little mushy, but you learn to anticipate them pretty quick. Who knows, it could just be my connection. The video is currently 720p only, but they are going to upgrade everything to 1080p once the service is up and running smoothly. I gotta say, its pretty awesome to be able to purchase a game and play it IMMEDIATELY, not have to wait for any downloading, and barely even any load time.
Its also sweet that they support all sorts of different controllers, keyboard and mouse, xbox 360, their own proprietary one, and others as well. (Not sure about the PS3, didn't plug my Dual Shock 3 in).
Its also really cool that you can, at any time instantly, spectate other people playing games. Right now on my other monitor, I am watching some dude play the beginning of Assassins Creed II.