I tried sourcing my
.vimrc in it, and it worked pretty well without many changes. That's really Neovim doing the hard work, but I wondered about GUI glitches from my colour scheme, syntax highlighting, and plugins.
I haven't used Neovim.app for real work yet (I use tmux/Vim most of the time), but a few things have impressed me. The startup time is good, the Dock icon looks great, and it hasn't crashed so far.
If you've got time to try it, follow the instructions in the readme for getting the Neovim through Homebrew and then get the source and run
make. You can run it by typing
open build/Neovim.app or by clicking on the icon from Finder.