I installed Gametap to play the one of the free Laura Croft games last winter. I found it straight forward and non-invasive. I guess they have free runs of games for several weeks at a time.
I also ended up trying a few other free games while I was at it. I've never tried the subscription service but I liked the fact that it doesn't sit in your tray and suck up resources and that the games that I tried downloaded and installed and played flawlessly.
I'd suggest trying the free version to get the feel of it.