I also tried Emacs.app and Aquamacs. Aquamac's behavior was totally odd & didn't match any of the documentation. I found the Cocca Emacs.app to be much easier to set up and get running. That said, neither of them feel as good as MacVim, as far as native Mac apps go.
The one that drives me really crazy is that on Lion, both Emacs freak out if you try to resize from from anywhere except the bottom right corner. Similar issues with the green (+) button; full screen mode, etc.
Also, since you'll probably remap cmd to meta, you lose some system shortcuts which can be convenient when switching back and forth between various apps and your brain can't keep all the shortcuts straight.
