Trust me when I say that if you use .NET, you're always going to be stuck with Microsoft. Mono, the only other somewhat viable implementation of the CLR, just doesn't really cut it in practice -- at least for ASP.NET.
That said, Mono benchmarked about 4x slower on my web apps.
For me, better to use two Windows instances rather than deal with the ongoing Mono compatibility drama across 8 Linux machines.
Not that I blame them in the least. I've been a huge fan of Miguel for years, and they're doing great things in the mobile space. I just can't in good conscience invest heavily in Mono knowing that it's essentially at a dead end -- particularly when other much more attractive web technologies have been released since .NET's inception.
The reality is that Microsoft never really wanted to build a cross-platform CLR. They wanted a great Java-like runtime that only works on Windows. If that matches up with your goals, then by all means use .NET, but be prepared for a tough slog later on if you want to escape Windows.
They never got the full stack running on the server and they punted most of the Windows-specific client stuff from the start.
They landed on a super smart subset and seem to be kicking ass with it. A C# compiler with some odd omissions and cool enhancements + native bindings to iOS and Android equals a damn useful tool. If you're building .NET or even Java backends it's certainly a very sane way to hook into them from Android phones and tablets in the enterprise.
But it's not a cross-platform .NET environment by any stretch and certainly isn't on the path to becoming one.