It's called wxWidgets now; Microsoft's lawyers weren't happy with the old name.
Its biggest problem -compared to Cocoa, Carbon, Win32, and even Qt, is that it has no standard method of data-driven GUI building. Cocoa and Carbon have nib files, and Qt has its own forms concept, but wxWidgets has none of these. The closest thing out there is wxGlade, but this requires add-on modules, and that introduces additional dependencies. This is most of what makes it such a pain to develop for.
Win32 has several methods for data-driven GUI building, with XAML and Windows.Forms being the latest. There is no One True Standard, unlike with Cocoa and Qt, but since Microsoft bundles them all with Windows the problem is almost moot.