How does it compare to an IDE, like Eclipse or Visual Studio (being the two I've used)? Is Atom an orange and an IDE an apple?
I do notice the occasional performance hiccup -- I'll try jumping through settings and the application will hang for 20-30 seconds. I would say this happens probably 1-2 times per day. Other than that I don't really have any complaints. The package/theme community is vibrant and updates/improvements arrive frequently. Sublime is still a more stable and capable application, but given Atom isn't even at a 1.0 release I think that's acceptable. I think it's only a matter of time before Atom overtakes Sublime in terms of stability and performance -- the current velocity of V8, Node and Electron have it well on its way.
However, neither integrates with a debugger like a traditional IDE. You'll likely drop to a terminal or external tools to run / test / debug your program. (I'm sure they both have a plugin to make terminal commands a keyboard shortcut away, though.)
Hope that helps. It's a fantastic editor, although it can be a bit RAM heavy on my aging MacBook Air.
But right now, it seems like its basically on par with ST, just with higher growth potential. That's not worth the switch for me at this current point in time.
It in no way replaces IntelliJ. Atom doesn't host SBT. It won't compile in the background, it doesn't parse your code and build an AST. It doesn't execute tests, etc etc.
With that out of the way, it seems like a really nice text editor with the really bad UX of shoving most commands into a "command palette" (which seems somewhat popular these days, for reasons I don't understand) instead of just giving you shortcuts to the common ones.
In practice this means I'm typing CMD+SHIFT+P to open the palette, type "grammar" to open the language switcher, and type "scala" and enter to select the language. Every time I create a new file. It's passionately lame. Also, "grammar"? As opposed to using the muscle memory everyone whose used a text editor any time in the past two decades has developed for "syntax"? What jerk thought that up?
But maybe I'm just missing something.
Also, because it's a cross platform node.js thing, nothing is native and all configuration is done in JSON ala Sublime. If this was a Windows application they'd be making you set all your preferences, even changing font size, through an .INI file.
Of course there aren't a lot of native apps that do a whole lot better IME. Textastic is great, but it's syntax parsing is lacking. ChocolatApp never bothered to get visual selection right. Sublime suffers some of the same faults as Atom and does the `subl .` work to open a given folder on the first try without Sublime already being open yet? Textmate is dated. Visual Studio Code is actually probably my favorite, but it's just Atom with a different skin and command palette and it doesn't support Scala yet.
I dunno. I flop between different editors weekly it seems. Maybe I'll give UltraEdit a shot. I liked that on Windows back in the day and it seems like there's a Mac version.
My 2c.
EDIT: Nope. UltraEdit is the worst of the lot. Feels like it's running under Wine. No Theme support (you have to choose background colors, highlight colors, etc manually), no Scala support that I can tell. You'd have to be a forever-time UltraEdit user on Windows to want to subject yourself to it on a Mac.
And debugging tools that actually work. A strong debugger has always been the reason Visual Studio has excelled, and they've brought that to an open source editor.