My family had photos taken at OKW back in September and I was very happy with the results.
As dbond said, they have access to most all locations on Disney property EXCEPT Disney / MGM Studios. You get the photographer for an hour, and they will take as many photos as you want, or they are physically able to in that period.
In the Magic Kingdom, they use the rose garden and get the castle in the background. That location makes for nice photos with immediate Disney recognition, but that's about the only location in the MK they can use simply because of the pedestrian traffic.
Epcot is a popular spot as they have access to the World Showcase before it opens. Time permitting, you could take photos with 2 or 3 different "lands" as your backdrop.
The lobby of the Grand Floridian is a popular interior location.
We decided to have our photos taken right at Old Key West, our resort for the week. We chose both an interior and exterior backdrop. All said, they took 40-50 photos in about 45 minutes. Obviously not all of the photos were suitable for framing, but we did receive all of the proofs. The best 16 photos were sent in a hard-bound, fold-open album that was embossed with Disney logos.