Re: Do any major apps actually use this?
The biggest problem with Xamarin (which may or may not be an actual problem in any given use case) is that it is mono based, meaning that every application incorporates an embedded mono framework runtime,
Also, Xamarin does not offer 100% portable code. In real world use 75% common code is the most you can expect. Even this can sound appealing, but beware lies, damned lies and statistics.... "common code" is all well and good, but the *TIME* spent creating the platform specific bits will not be proportional to the amount of code that is portable, or not. And one has to suspect that things like manifest assets etc are most likely being discounted/ignored (it's not "code" right ?) for the purposes of establishing a flattering statistic.
Why would I question the appeal ? Only because I know people who have used Xamarin and who found it's delivery fell spectacularly short of it's promise.
I myself use RemObjects Elements which takes a radically different approach.
They offer no common runtime or framework, choosing instead to provide common *languages* which extend directly into the native platform frameworks. They offer C# and Object Pascal currently with a Swift compiler coming soon (currently in beta). You can mix and match all of these languages in a single project and that project can target .NET, Cocoa/CocoaTouch or Java/Android.
You can share code across the platforms with some nifty type mapping technology in their compilers which maps low level constructs expressed in a standardised dialect, directly onto underlying platform specific constructs. i.e. you can use #integer#.toString() consistently across Java, ,NET and Cocoa, rather than having to use platform specific conversions. For example.
Or you can stick to using platform specifics, if you prefer.
The advantage being of course that your resulting apps are indistinguishable from the truly native platform SDK products, with no runtime overheaded or embedded framework required to prop them up.
My most recent RemObjects produced app weighed in at < 40KB on Android, 100KB on iOS (thanks to embedded launch screen requirements stipulated by iOS) and 60KB on Windows Phone.
TXT-2-PARK is the app, if anyone is interested (only relevant to New Zealand readers I'm afraid, being an app for use with SMS based pay-and-display parking meters here in the Land of the Long White Cloud).