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.
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.
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.