It's extremely similar to Sublime Text, ST is clearly the model Github is basing it off of.
It excels over ST in package management and discovery (built right into the app with a very nice UI), much better project wide search, and I find it is easier to hack on.
Sublime Text so far is better at multiple cursors (although Atom is fast catching up), is a bit faster over all, and its fuzzy file search is better. But, I don't think it will be too long before Atom addresses all of those and more. I have no doubt Atom will ultimately make Sublime Text irrelevant.