At AK, they are in Africa just near the entrance to the Kilimanjaro Safari, and they are totally worth it!!!
We had both our 7 and 3 year old daughter's faces painted (they range from $10 - $15 depending on the design). They were awesome...
Even better, about an hour later we made the mistake of going on Kali River Rapids (mistake not because it was not fun, because it is, but because they both got very wet and ruined their faces)...
We took two very sad girls back to the face painters to see what they could do, and they more than happily redid them (they called it a 'touch-up' but they pretty much re-did it) and charged us nothing!! They apparently charge $5 for a re-do, but I can't imagine what that entails since they washed both my kids faces and started basically from scratch.
In fact, we went by about 5 hours later to go on an evening Safari (something I definitely recommend) and they weren't busy and the one CM offered to touch them up again (which they didn't need)...
In short, the CMs at AK painting were great...I know most if not all the designs are the same at every park (I think the Dinosaur face is only at AK, but I may be wrong).
I normally wouldn't pay for face painting at a fair or something, but it is definitely part of our Disney tradition now...