I personally think it's still worthwhile learning the Win32 API.
As far as I recall when I started learning Win32 (after doing some VB(A), Pascal, etc.) I learned a lot about Windows and understood how thing works in Windows. Everything was so clearer. :)
So, as per your question - you will learn a lot about Windows through learning Win32.
As you said - you're a C# programmer and I'm not sure if you'll use it often, because almost everything you need is already there, in .NET.
