WPF is a radical overhaul for the next generation of Windows graphic
UIs; because it isn't bound to the Win32 control-set, it can have a
lot more flexibility. It has good support for media and 3d display,
although it can also be used for simple UIs. It also offers good
scaling support etc. However, it obviously demands .NET 3.0, and works
best on Vista with a reasonable GPU - although it backports to XP; not
sure about 2003 etc. I suspect (without evidence) that it also places
more demands on the system (although it may actually be better-able to
offload a lot to the GPU).
I severly doubt that it will ever completely *replace* winform, but
for anything where you want it to be visually appealing, it is a very
enticing option. Unfortuantely I mainly work on internal business
apps, so I doubt I'll see much payback from taking the time to learn
it...
Aside; because the xaml model is similar to that used by xbap/
silverlight, it might be useful if you are also dabbling in those
areas.
And since it is possible to host WPF in winform, and winform controls
in WPF, it isn't "all or nothing"; you can mix and match.
Marc