I put my name in the hat (and received) for the free year long membership they were giving out. Thus far I have been unimpressed. The service requires a 5 MBit connection (the documentation when I signed up said you only needed 3MBit, which is what I had at the time). When you attempt to log in, the service wont let you connect if your speed is below 5MBit. Additionally, all but one time I logged in I was told my latency was too high for good service (I have ATT Uverse and most times I get excellent ping times for servers), but at least it lets you log in.
For their game selection, there are currently only about 10-12 games, for which you have to buy or purchase play time, in addition to any other service charges you might be paying. Most games have a free demo option. I tried Lego Harry Potter. The game launched just fine. Lego Potter starts off with several clip scenes. A minute or two into it, I started getting video corruption, like when watching a video file with the picture freezing but then the changing pixels (i.e. where movement occurs) continuing to update, slowly refreshing the screen. I gave up on it before I even got to the gameplay.
My potter attempt was on one of the instances where I was told my latency was too high. (The service doesn't tell you what your latency is, unfortunately). The time I managed to log in without getting a warning, the Onlive interface started doing the same video corruption that potter had done. (The interface has little animated game windows and sliders and what not.)
My opinion would be to avoid the service for a few months, and hope that things improve.