First, let me say that I'm really exited about the work you guys are doing, and I've been following for quite a while. When I heard about the partnership with Microsoft and Visual Studio, and the move by MS into cross-platform CLI, I finally decided to give the Xamarin platform a real try. However, thus far I'd have to say I'm a little disappointed.
In my opinion, the platform is still at a level where much of it should be offered for free, especially with a paid MSDN subscription. Although there is a free version, it is pretty much useless for even the most basic of discovery and prototype activities due to the size limit restrictions and the like. Moving past that, here are a few main reasons this platform is not quite ready for "prime-time":
No Visual Designer for Xamarin.Forms:
The whole platform is predicated on C#-backed XAML, which as we all know is a visual mark-up language. For me, a WYSIWYG editor should have been "job one" in the backlog. I would understand (sort of) not having this for VS, but at a minimum your own IDE should have one. VS has a Visual Designer for XAML that works with MS forms, so I would think this would not be a far stretch save for the device-dependent visual models, which should be abstracted. Maybe a good reason to move to your own OpenGL-based renderer Of course, there are pros and cons with either.
No Windows 7 and Windows 8 Desktop Build:
The only way to get a desktop build currently is on Mac OS-X or Windows 10, which is not even officially released yet. Being as the most optimal power-house development will be in Visual Studio, this means that you have to do very long builds with all their associated packaging and then do your testing directly on Android/iOS devices and emulators. Coupled with the fact that their is no Visual Designer, this makes UI iterations fairly painful for a forms-based technology. I've seen some old posts on WPF integration, but nothing solid.
No Visual Studio Wizard to add a Windows build:
You have to do this manually through the generic Windows App wizard, as described here: http://developer.xamarin.com/guides/cross-platform/xamarin-forms/windows/getting-started/tablet/ . Even after this, the resulting project will only build on Windows 10.
Again, great work, but it's a little hard to justify a paid subscription with such large feature gaps, and in my trial of this product thus-far I have struggled to get anything besides an Android app to work, as there seems to be many manual steps and external dependencies otherwise. This also in light of the fact that Qt / QML covers all these platforms and more for free, and has a Visual Studio integration as well.
Keep up the good work! I can't wait until we all can forget about the OS and device wars and truly focus on bringing our business value to market...
PS - Your installer should not brute-force download the Android SDK. Visual Studio does the same thing. It is likely that someone using these products already has been doing some mobile development and has Android installed (along with Ant and the JDK!), and you just blew all that away and destroyed their current environment by not letting the user simply reference what's already there.