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I use Mono on a daily basis. It's a mature full featured cross platform .NET implementation.

It's not full featured. It's a pretty small subset. The most powerful things in the .Net platform are the big chunks of code which take on large business cases and make them go away.

For example the following either don't exist or are half-baked:

WCF (WsHttpBinding in particular), WWF (entirely which we use for WCF correlation), EF (not a biggy but would be nice), WPF (the best part of .Net), MEF (buggy and missing bits).

If I wanted cross platform, I'd go Java.

I do agree with you : mono is a subset of .NET, Cross platform compatibility is not the main reason for using Mono. I really like the C# and the .NET ecosystem. I use a native UI for each platform, but the core code is the same. I'm not a big fan of those generic UI toolkit ...

Agree on generic UI front!

Not true: http://www.mono-project.com

There's Mono + Qt bindings.