I've had it for about 6 months with mixed feelings. On one side, it's great to be able to play games I'm interested in, but don't want to buy. AKA short single player games with limited replay value. On the other hand, I never get new releases until a month + after the release date. If it's something you just have to play the week it comes out, you're better off buying it or renting it at a Blockbuster store.
I'm probably going to cancel it next month when Killzone 3 comes out. After that there's a steady stream of games coming out I plan on purchasing. I only got Gamefly because the middle of last year was very slow for games. This year there's something good coming out almost every month (if you're a multi-console owner)