I've used Xamarin for several projects and I greatly regret that decision. Granted, most of my hatred is targeted towards Xamarin Studio and Xamarin.Forms so if you use Visual Studio and build only native UIs then maybe your experience will be better.
However, in my experience, both Xamarin Studio and the build system are buggy as hell. Random or inexplicable build errors, things that break during updates, incompatibilities with official Android support libraries... I find myself doing 'clean project and rebuild' to fix random errors, or switching between alpha, beta and 'stable' channels all the time depending on which one does not have the bugs that I'm running in to.
Xamarin.Forms is simply a disaster. Because it aims to unify the apis for the UIs for various platforms it boils down to only the most common denominator of those platforms. And then makes it worse. Not only is it buggy, it is also very slow and incredibly limited. In our office we're keeping a list of all of Xamarin's silliness we encounter, here is just one of those:
"Clicking a Button changes its text alignment from center to left-aligned; it requires writing a custom Renderer to solve this."
I admit that Microsoft is usually quick to fix those bugs, but it doesn't instill much trust in the system if you're constantly running into issues. Many days I am literally working 50% of the time on my app and 50% working/fighting my way around Xamarins issues.
I'd love to hear from someone using Visual Studio if their experience is more positive, but my advise is: please stay away from Xamarin.Forms and Xamarin Studio as much as possible.