Qt is way too bloated in my book, and it can be rather unintuitive at times. It gets the job done though, so if wxWidgets isn't up your alley (I can't say, having never used it) you might give it a try. I think the difference between professional and open source shows pretty clearly in the design of all those GUI toolkits. For instance, I've always found that Windows Forms was very easy to develop for and that it was very intuitive. Cocoa is very nice, too, although I don't really like Objective-C. It's a shame that these are platform-specific.
By the way, if you end up writing it yourself, you might want to try C++/CLI (a "managed" variant of C++) for your Windows GUI. It lets you use the .net Framework in C++ and although the GUI will end up being "managed", you can easily call your own "unmanaged" C/C++ routines. (You could e.g. also write the GUI in C# and write a wrapper for your C++ code in C++/CLI.)