I have been twice with two different birthday boys and we really enjoyed it. It is about $10 per person.
It is located outside the Plaza Inn, and everyone enters at the same time. You check in at a kiosk and then wait in line to enter. I think the best tables are the ones closest to the dance floor and with an UMBRELLA.
Each person will get an individual cake with their own frosting and decorations and some punch. You also get a party hat that looks like Mickey's Sorcerer hat and I think a noisemaker. The party is hosted by Pat E. Cake, who sings some songs and kinda gets the party going. After the cakes Mickey and Minnie show up for the birthday song, a conga line with the birthday people, and then a photo with each.
It is a lot of fun and very cute.
I dont know for sure that its still there, but I havent heard anything about it going away and its always so popular I cant imagine they would. The only complaints I have is that that area is VERY hot in the afternoon when the party is, and getting the professional photo afterwards takes forever.