I haven't subscribed to Gametap, but I have downloaded their software for a free account and I have used the system for a few weeks...
You use their client to download the game, and every time you want to play the game you start the Gametap software, and you can start the game from there.
I basically dl'ed it so the kids would have some new games to play every few days... they can try Megaman, Jaws Revenge, other old arcade titles from years ago.
If you subscribe (which I haven't) you can access games like Hitman, Myth II: Soulblighter, Sid Meier's Alpha Centauri, X-Com: UFO Defense, Myth III, Civilization III Gold Edition, Command & Conquer, Homeworld I and II, Heroes of Might and Magic I, II and III, and Empire Earth.
Now the real question is: Is it worth $60/year?
That question is a little harder to answer.
But the system seems to work fine and I haven't had any problems with it.
Hope this helps....
APCFreak