Carmax?

We had a good experience with Carmax-granted it was in 2005.
The cars we needed to sell were a 2003 Mazda MPV and a 1995 Toyota Corolla.  The price given for the MPV more than paid off the amount that we owed on it.  We didn't get much for the Corolla, but it had been through a bit. 
As an aside, I cried when I turned in the keys for the Corolla....I loved that car. 

I think its worth going to get a price for your car so you know your fall back price.  I have used them a few times to both buy and sell.  In DFW we actually got pretty good prices but in NY the figures were bottom book so we sold privately.  In terms of driving in and walking out with a check its easy - but as has been said with a little effort you can usually get more!