Everything in this world has a place, so does the surface. Not everyone can afford the biggest mansion on the hill. For the few that can. I'm happy for you.
What you mean by "unusable GUI"? The same button you clicked to get to this page looks 99% and acts 100% the same on every platform. A button is a button. A textBox is a textBox no matter what any platform call it.
This post inspired me to fire up my Linux mint virtual box and test mono. I'm every impressed by its potential. It does lack some awesome c# features like async/await. I've been using Xamarin for a while now and love it. My only fear for mono on Linux is the lack of XAML like interface equivalent. I dislike GTK. I'm keeping my fingers crossed :)