I used Gametap for a couple of months a few summers ago. All in all, it kept me entertained for a while and didn't cost too much money. Downloading games over their service was painfully slow, however. A 1 GB game would take all day to download through them. I can download the same from a fast webserver in 10 minutes. Also, quite a few of their games did not work on Vista, even though retail versions of the same games had be patched by the developers to work fine. For the games that did work with Vista, and after the long download, I spent many hours playing a ton of games.
I stopped subscribing before their current tiers of service, so I can't comment on what you get for what you pay anymore. When I did cancel, they emailed me and offered me a free month if I stuck around. After that free month, I cancelled again, and they emailed me to offer me a 50% off month. School was back in session, so I declined.