I've been generally satisfied with them. I've been a member for over a year, and I do like that if a new title in my queue comes out and I have an open rental slot, odds are that I'll get the game shipped out that same day. The "Fast Return" feature is nice when it works, and I've only had issues with 2 games not working from them.
That all being said, it is not without it's flaws. Being a Netflix and Gamefly subscriber, the added 1-3 day delay I experience with GF that I don't with NF gets frustrating sometimes. Also, sometimes games seem to take an extended period of time to be returned to them. I live in CT and my nearest shipping center is in Pittsburgh, and a few games have taken nearly 5 business days to arrive, typically at the least convenient times. Finally, the service sometimes gets confused with multiple games out and the fast return.
I have a 2-game account. In preparation for last week's MadWorld and RE5 releases, I had 1 slot open from the previous week and 1 game out that I was going to send back on Monday. Theoretically, MadWorld should have shipped out from the open slot, and RE5 would have shipped out after the Monday game was returned, which would likely have happened via Fast Return. INSTEAD, MadWorld shipped out via Fast Return, which only applies for games that are still in transit. The open slot essentially disappeared and RE5 wouldn't ship out until the game in transit was received. Of course they didn't get the game in time and I was stuck without RE5 even though I had slots prepared for it.
So essentially, it is good if you know how to use it and is getting better as they improve their methods and add new shipping centers.