Atom is still in beta (v0.123 as I'm writing this) but it's moving fast. Way faster than Sublime. New builds are released on a weekly basis, sometimes even few of them in the same week. In its short life span, it had more releases than Sublime which takes months to release a new feature or a bug fix. Here's an updated take on things looking back on the path Atom has taken since the launch of the beta:
Sublime has better performance than Atom. Simply because it's written in C++. Atom on the other hand is a web based desktop app built on top of Chromium, and while they take performance close to heart, it will be really hard or even impossible to reach the same speed and responsiveness. Last July Atom began using React and it gave it a nice performance boost but you can still feel the difference. Apart from that, if Atom’s performance issues will not push users away - Sublime better speed up the release cycle, brush up its small UX tweaks, and consider letting in more contributors because this is where Atom is winning.
Atom's package ecosystem is also growing really fast, it might not be as big as Sublime's at the moment but I have a feeling that with GitHub at it's back it will keep growing even faster. It probably has the majority of IDE like plug-ins you can think of. A major difference right now is that it can't handle files bigger than 2MB so it's something to keep in mind.
The one thing you'll notice first is that the Sublime minimap is gone! Other than that, the first impression is that Atom looks almost the same as Sublime. I wrote a more in depth comparison about it in this blog post.
No easy straightforward way to port your Sublime configurations, packages and such as far as I know.