I've had Gametap for a few months and I've never encountered any outages. It works very well for playing old and new games. Some of the older ones, like Kings Quest, run in windowed mode though. It is completely transparent when you are in a game.
I haven't played Xcom on the service, but I always had issues running it in dosbox, mostly just with the speed settings (like the scrolling speed.) On Gametap, I've never had any issues running older games, they are programmed to be playable within the Gametap client. So yes, there is a client that all the games run on, and it isn't very intrusive. When I want to play a game from Gametap, I load up the client and launch it directly from the service. The Gametap client runs much like an internet browser.
It's only 99 cents for the first month, if you are interested in giving it a try. I think it's well worth that.
I'm also surprised at the newer titles they've managed to get a hold of. Jericho, the Sam & Max series, and Kane & Lynch are among the few that are included with your subscription.
They aren't paying me to say this stuff.