How to write modern Windows software in C++?
From what I've seen over the past several years:
WTL is on a lifeline. Abandoned by Microsoft, picked up by fans and there were several very dedicated followers. Very clean, but the learning curve is steep and the fan base dwindling. The Yahoo group is not very active. I can't recommend it.
MFC got another lease on life when MSFT released the feature pack. Rather extensive and a bit un-MFC-ish, it has strong support for skinning, docking layouts and ribbons. I thought it was going to be wildly popular but never did see a lot of devs jumping on it. MSDN forum questions have been sparse. If you have an existing MFC codebase then definitely take a look. Another MFC refresh for VS2010 with Win7 features added, it does stay the company's base UI solution.
wxWidgets is still around. No personal experience, but Lord, the few practitioners I've heard from are bitching up a storm. Real bitter stuff too.
Qt has been around for quite a while but picked up stream considerably, especially in the last year. Whomever uses it really likes it. It is moving beyond the confines of a UI class library as well, their users are looking actively for solutions to common programming tasks that start with the letter Q. That's a powerful vote of confidence.
But if you are on a Microsoft stack, none of these class libraries is where the real UI development is at. WPF is the elephant in the room, it's capabilities are a hundred miles beyond what's listed above. Its ability to break device and paradigm boundaries are powerful, writing code that runs on a desktop as well as a web browser as well as a phone is hard to beat. But C++ is not part of that.
WTL, Qt, wxWidgets are used pretty common.
They all allow rapid GUI development and are pretty versatile (third option, to my mind, takes more time to get used to). Second and third options are really cool because they're cross-platform.
Writing windows in pure WinAPI is extremely oldschool but sometimes could be entertaining.
C#approach to building windows is even more rapid compared to previous options, but is a bit less flexible.