DH and I love the Rainforest, and often visit the one in Nashville, since that is the closest to us.
We have never eaten at the Rainforest at the Animal Kingdom, but I imagine it may not be as busy as the Rainforest Cafe in DTD. If you want to eat that the one at DTD, I suggest putting in your name early - it is not uncommon to have a 2 hour wait in the evening - afternoons usually aren't as bad, but again, it can be hit or miss.
We love the animatronics inside the restaurant, and the food has never been a disappointment!