I used them for a few years in the past, then I quit for a year, and started my account again just recently. Their turnaround times on games has vastly improved in that year, from a week and a half to two weeks per down to 4 days on average. It's by no means anywhere near netflix's service, but it suffices.
I haven't had a problem with disc quality yet. I've never had to send a game back to them because it was so scratched it was unplayable.
Like it was said, if you expect to get new releases, good luck with that. I've had them skip over the first 6 items in my gamefly list, when the last four were marked "High" meaning 75-95% chance they will send that game, or whatever their arbitrary measurement system means. They're always understocked, and their measurement system for availability should be updated somehow.
If you're like me, and really just want the luxury of always having a "new" game at your house, I'd go with the two game plan. I keep my game queue filled with past and present titles I want to play, and I always have at least one game at my house at all times, often times, two. I get bored with games quickly, so gamefly really helps me cut down on my spending on games.
They have other incentives too, discounts on buying used games, and the ability to trade in games toward your monthly subscription.
Overall, I highly reccomend GameFly. However, if you like to find every secret, and gather every achievement, you may not find your money well spent there. Basically, with the two game plan being 23$ a month, if you go through at least one game per month, the subscription has been worth it.