Not sure how many out there are using alternative IDEs, but I discovered the Omnisharp package for Github’s Atom editor earlier this year and (with a little finagling) managed to get it working with Unity. I’ve been away from Unity for a few months now, though I’ve kept it updated to 5.3.X and now the most recent 5.4.0 release.
I had previously set the external editor setting in Preferences > Editor Tools to use Atom instead of MonoDevelop and selecting “Open C# Project” in the Assets menu opened the entire project in Atom without issue. Now, however, choosing either the “Open C# Project” or double-clicking on a C# script will open both Atom and MonoDevelop and Atom will fail to open the project. Regardless of whether I set the default editor back to MonoDevelop before setting it back to Atom, Unity continues to open both IDEs.
Has anyone else experienced this issue? I have been able to replicate this behavior in both version 5.3.4f1 and 5.4.0f3.