I have done this several times. The location is at Innoventions West in Epcot. The times are 1pm-4pm (or 1:30-4:30pm). The characters change. I believe there are always 2. I've had Mickey and Pluto twice and Mickey, Minnie and Goofy once. It was a good experience overall. If you go right when they open the line can be long. But if you wait until like 2 or 3 the line is short. Maybe 10 min. It can be annoying to have to stop to pick up your picture on the way out, but hey, it's FREE!
I have also noticed that the lighing in the room isn't great. Doesn't make for the best quality picture. But again.... it's FREE!!