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.
My 5 year old macbook pro handles the development version of jquery (10,309 loc) with syntax highlighting just fine. It can be (a bit) janky when scrolling through the whole thing, but it's not very noticeable. Local edits / search are as fast as vim.
