IronPython with WPF is the most productive way to design a GUI that I'm aware of. Almost all of the app pictured below is written in IronPython. Only very small parts are done in C# or C++/CLI for performance reasons.
Sure, you do miss some WPF/C# integration, but you gain much more on the productiveness side.
IronPython is also much easier to extend than CPython. You take your C++ code and just add a small C++/CLI wrapper on top of it. CPython has nothing which is as easy as this. That's the reason that I've started using IronPython, because I got tired of writing wrappers for my C++ code in CPython. Boost.Python, SWIG, and the like didn't work for me.
