Sublime Text user here, who switched to Atom a year ago and never looked back. Never had any of the issues others described having with Atom such as slowness, etc. What I love about Atom is that it's under active, fast-paced development, that it's super-easy to customize (first thing was to increase the font-size of the left-hand folder pane), and the available plugins are fantastic. It probably also helps that extending Atom is much more approachable given that it's all Javascript, rather than Python with ST.
Furthermore, as others commented, the plugins have much more reach into the editor and can do a lot more.
For example, the new block decorations coming in 1.6 are going to be an awesome API for plugin authors! http://blog.atom.io/2016/02/03/introducing-block-decorations...