I've been using it on and off since the initial release. I'm not particularly impressed by it so far; in particular, the performance relative to ST3 is absolutely appalling in every respect. It's still in development, I understand - but one of the key awesome things about ST3 is how amazing, earth-shatteringly fast it is.

What I'd like more than Atom would be an evolved version of Sublime. There's still a little bit of cruft in there from what I can see - while I've never developed a plugin, I've heard it's a bit tricky. I'd love something with proper support for things like SublimeREPL more extensive capabilities for previewing documents, and perhaps better ways to theme and write plugins using alternative languages.

I'm not totally convinced that Github are going to be able to make Atom a compelling offering on the whole.

Atom.io is too slow for most Sublime users, especially on hardware that is not bleeding-edge.