I haven't seen Brunch before, but it still seems to suffer from the same major flaw as Grunt, which is no sense of file-file relationships:

"Brunch recompiles and concats all your stuff automatically on any change, headlessly. No more need in compicated Makefiles and watchers."

Perhaps I'm wrong, but Brunch just seems like a variant on Grunt with a lot more out of the box for building front-end web applications.

Still, it does look cool. Thanks for the link!