At first I was under the impression that the subscription would cover the cost of the games, much like netflix does for movies. It sounds like this is not the case so forking over 15/month doesn't sound like such a good deal.
If you have to buy the console and the games, and they're priced similary to other offers, the monthly $15 would buy you, I guess, the same as Xbox Live, the online experience/friends/matches (which people do pay, afterall).
Now if the OnLive "console" (receiver?) is cheaper, it's a different story...
xbox live is also much cheaper and isn't required for single player games. An xbox arcade is 170.00 dollars new - the same price as 11 month of the onlive service and you don't have to maintain a subscription to keep access to the games you have bought, yet you have the same advantages as onlive in terms of not having to upgrade your hardware every year to play the latest game.
With this... all you're getting for your $15 is the privilege of playing games that would otherwise not cost extra to play on other platforms.
It's a little bit like the Steam argument - Steam DRM is "worth it" because it confers features you wouldn't have otherwise (no more disc juggling, serial numbers to remember, etc), whereas normal DRM is just pointless.