I have tried it, and here's my thoughts.
It's major problems are three things.
1. Visual quality. It is like JPEG versus BMP/PNG. The compression is bad. It isn't consistent either. Static scenes will look more clear, ones in motion, less so. Regardless, it always look worse than the raw thing. A foggy blur like appearance covers everything (edit: reading that link, "washed out" is a good way to put it).
2. If they ever go under, you lose access to use your purchased right to the games, but you won't get your money back (technically, the same is potentially true of some others, like Steam).
3. All of the general extra things that come with PC gaming (modding namely, etc.) are lost.
Also, supposedly done away with, but worth noting, if they ever decide to start charging for this service, it COMPLETELY changes it from an alternative to a needless and inferior one. The money you repeatedly pay for service would nearly get you a console or PC to play better.
The selection is limited right now too, but that will get better in time.
The things I do like are as follows.
1. There's no need to install any games. It's pretty much plug and play, per se.
2. Requirements are a non-issue, sort of, but you still need a good connection and a PC capable of HD video playback (to a degree).
3. They usually offer all games in sale format of full (full price, full access), rent (full access for a set amount of days), and trial (full, yes full, access, but for thirty minutes only). When that thirty minutes is up, you can go back and play it again, so it doesn't "run out", but you obviously have to start anew.
Ughm... about it really. Those pros aren't issues for me against playing games the usual way, so I don't prefer OnLive. It's "neat", but if this is ALL we had for gaming in practice, we'd be in a bad state. That last statement summarizes my thoughts about gaming this way, and thus, OnLive.
