Comparing Gamefly's trade in program to ebay or gamestop doesn't really add up. Since you receive credit on subscription fees, I assume you'd have to be a member. If you're a gamefly member you're probably not going to gamestop much for anything.
To me it just another reason to ditch buying new games or hunting around for used games, and go with a single, convenient source.
If I don't have to drive to gamestop just to find out that they want to give me next to nothin...
It got too expensive for me. I bought an xbox360 for a couple hundie and added a gamefly subscription for $20 / month. Low investment, minimal ongoing costs, but lots of killer gaming.