I like wxPython here's why.
- It's cross platform, although I haven't used it on other platforms (besides windows) and I've heard a few bad things, but not much.
- It's supported by several programming languages. I'm using it for Python but I could be using C++ (which is native) or wxPerl, wxLua, wxJava, wxNet, etc. This means that as I learn the details of the library that knowledge is transferable.
- wxWidgets and wxPython are very active.
- There is some activity for small form factor versions like for Palm, and WinCE. I think the next big platform is the cell phone. Like the Nokia series 60 which supports Java and Python (list of phones).
- It's very close to Microsoft's MFC and supports the Windows platform very well. I used MFC for a long time and became used to it. wxPython is like putting on an old pair of jeans. This is also a negative, for example, I never liked Microsoft's ListCtrl very much and it's nearly identical in wxPython.
- The wxPython community has created a bunch of samples which can get you going very quickly.