I'm a few weeks in to using Xamarin Forms and I have to say, the idea is amazing and the allure is strong. I think once Forms matures you're going to have an absolutely killer thing here.
I'm just having a really hard time getting past some extremely frustrating aspects of the platform. First - read that again. You are building a platform here. So why is everything so locked in with internal modifiers, lack of overrides, etc...? I get the "not officially done" aspect of the API but at the same time you're telling people to make custom controls and renderers. So which is it - do you want us to do custom stuff or not?
Second frustrating part is - who are you targeting? Forms is built around Xaml and C# so presumably you're targeting people with at least some previous WPF/WP8/Silverlight experience. But the Forms API is far enough apart that they're going to have a hard time jumping in. People coming from Android and iOS are going to have a learning curve either way - why not make it easy for at least some of your user base? Additionally for the Android and iOS crowd they aren't going to have much learning carryover to MS Xaml so it's not like they're learning that native language either.
Third - if I need to do custom controls I have absolutely no idea where to start in the API. Do I need a ContentView? A Layout subclass? A Layout subclass (which btw, why generics?)? The documentation doesn't really explain anything.
And finally - one of the best things about MS Xaml is the ability to separate out layout from views (from data). Take ItemsControl for example - it's a combination of a Panel (Layout), data source and templating. It doesn't render anything on it's own but it allows you to combine a set of views (templated data) to a layout. I've mostly recreated this but to make it work I need to have the ItemsControl itself be a view which means deeper nesting, more work for layout, etc... Not every Xaml element should be required to map to a View with a renderer. If I'm missing something and what I've just mentioned is possible then please do let me know!!
I really hope you don't take all of this as a sign I hate the product, I think it's fantastic. It's just frustrating to see (and work through) Forms going through the same growing pains that MS Xaml platforms went through circa 2007. Ok, end rant 