WinForms is a pretty great UI toolkit. It's easy to use, fast, and requires very little system resources.

This. It might appear a bit clunky at first sight, and it might not provide the shiniest eye candy out of the box but in terms of development velocity to get something practical done - provided it is combined with the tooling in Visual Studio - it's really nice.

Velocity? Only for simple, boring UI.

Any non-standard control requires GDI+ rendering, which is not very maintainable. WPF offers a far superior composition ability.

I agree I would not suggest doing anything non-standard with it. And yes, it looks dated and ugly. But in some places visual appearance of widgets is not that important.

Often, boring UI:s are good. The tool should fade into the background and let the user focus on getting her work done.

While I love shiny things the real world accepts good enough solutions that are economic to develop.

Theoretically I just adore WPF:s scenegraph oriented paradigm since it feels the right way but... I've observed that for a large projects practically it needs a lot of work to get anything usefull done compared to forms. Perhaps the overall architecture is a bit big-org oriented where every tiny widget will have its own development team. Which is understandable but means for simple UI:s WPF might be a lot more expensive. I haven't done anything massive with it myself but have just observed a few projects from my work. I hope there are counterexamples.

"Non-standard" doesn't necessarily mean "shiny". Often you need to compose elements in a way that makes a certain UI feature possible, or convenient. Such compositions aren't inherently shiny; they can even be ugly. But functional.

In WF it's very hard & tedious.

I love using simple boring UIs because they are predictable. WPF encourages non-standard custom drawn stuff that only serves to distract from usability in my opinion. WPF also still has weird focus rectangles and font issues.

Yes it is quite nice, but WPF/XAML is better.

Doesn't sound very greenfield if you're already forced to use an aging legacy technology.

I'm implementing a brand-new GUI application in Win32 right now, for three reasons:

1) It's going to be used in conjunction with other legacy applications that only run on Windows PCs anyway.

2) GUIs designed for both phones and PCs don't work well on either, it seems to me.

3) In fancy high-level frameworks you often eventually end up calling a bunch of Win32 functions, so I might as well learn it. :)

The application is greenfield, the environment where it needs to run, not.