One topical question was "what does using Xamarin and C# offer existing Android Java developers?". I have a couple of thoughts which are listed below (in no particular order):
Learn a new language!
It can be daunting though, to start from scratch and learn a new IDE, framework and syntax - which is why I think C# is a great progression for Java devs. The two languages are obviously related - there is a lot of common syntax and equivalent keywords that make it easy to get proficient quickly. They're both strongly typed, so the IDE and compiler can really help you learn. Then once you know the C# basics, a whole world of new APIs open up to you: LINQ, the .NET framework's serialization and web service features, Parallel Task Library and async support, reactive extensions, and more.
It also introduces you to another IDE (Visual Studio is a great tool, as is Xamarin's free MonoDevelop IDE), which in turn can encourage you to further explore other languages and platforms (suddenly it's a small leap to work on Windows Phone, or try F#). Using the free trial you can easily get productive in C# and Android development without any up-front cost: you can run your apps on the Android emulator, and choose to work on either Windows or Mac with MonoDevelop.
There are a lot of Java developers who are already proficient with the Android SDK and have published successful apps to Google Play, or Amazon, or Samsung app stores; and there's another group of corporate/enterprise developers building apps for internal/extranet use that were designed for Android. When these developers ask "why Xamarin?", I never say "throw away everything you've done and re-write in C#!" -- that'd be crazy ;-)
But what happens when you want to take these apps cross-platform, either to make more money on the iOS and Mac App Stores or to better support a corporate BYOD policy which means your internal apps need to run on iOS and Windows? You can choose to learn Objective-C to write for iOS and C# for Windows... or take the opportunity provided by Xamarin to use C# for all the non-Java platforms and reduce the amount of code you have to support and re-write by half!
Following on from the first point, I think learning C# is a much smaller leap for a Java developer than Objective-C; and once you've done so you can work on either iOS apps using Xamarin's MonoTouch or Windows Phone, Windows 8 and Windows RT tablet apps! You might like C# so much that you do go back and re-write your Java, or at least start building a hybrid that shares some Java and C# code...
Xamarin does Android well too :)
If Java developers do fall in love with C# then they also find that Xamarin does Android just as well as anyone (probably better!). Our MonoDevelop IDE (and our integration with Visual Studio) means you get an awesome coding experience (autocomplete/intellisense/code completion, debugging, refactoring, source control integration, etc) as well as a drag-and-drop UI designer. You be the judge of whether it's a better experience than Eclipse :)
In summary, why not? As a Java developer you have a lot to gain and nothing to lose by giving C# a try. A whole world of new platforms, tools and frameworks become available with a very low barrier to entry. Try it - you just might like it ;-)