Are there really that many people building .NET projects on non-Windows systems? It's been my experience that companies that are using .NET are pretty much full Windows everywhere.
Roslyn needs cross-platform MSBuild to build itself for every platform (without Mono). Same with corefx.
So even if you're currently using or thinking about using .NET xplat and don't have any plans to use MSBuild, this move is necessary for us working on .NET to get xplat builds working.
I'll definitely be. I live in the nix terminal, and that's where the tools work best for web development. But the current day job is C# apps. I dream of hacking code legitimately in a nix.
I'm writing Mac, Android and iOS apps using Xamarin in C# using a shared PCL that is 99% the same code across all platforms. This might not be all that common, but Xamarin has seemingly been gaining momentum over the last couple of years.
I've mostly spent time on the Mac side of things so far, and have helped out a colleague with the mobile stuff. Recently they've added a lot of documentation for the Mac side that covers basically anything you want to know - or at least it covered all the stuff I wanted to know when I first started the Mac app 12 months ago.
For mobile, Xamarin Forms can be a little difficult to wotk with, but it's getting better all the time. I don't really want to bother with learning native development at the moment, and because we have a pile of calculations and logic in our shared PCL Xamarin is working out for us.
There is some criticism that the pricing model makes it expensive for people developing at the low end of the mobile market ($1 or $2 apps), however I think that's more a failure of that kind of market in general rather than a problem with Xamarin pricing itself.
It's been the case so far because Mono feels like a second class citizen, but when MS will release officially .NET on other platforms maybe it will change.
My experience as well. I did build some older stuff back in maybe 2010 using mono and NAnt as a hobby of course and I still use NAnt today in 2015 but all it does is invoke MSBuild. These days I don't build any .net software on non Microsoft platforms.
So even if you're currently using or thinking about using .NET xplat and don't have any plans to use MSBuild, this move is necessary for us working on .NET to get xplat builds working.