@TommyCarlier: There's been come dramatic speed improvements over the last few releases. I also find the WPF aspect of live messenger pretty engaging and pleasant, though i find a lot of other things about that program to be pretty poor. I feel like WPF is a lot like flash, both technically and in a lot of practical ways. A lot of people who make flash or WPF applications get carried away with gratuitous animations and graphics which turns the program into a sluggish memory hog. It is possible to write great and performant apps with wpf. Metrotwit and live messenger 2011 are two of those. It's also not a long way from how people often say c# and java applications are all slow and terrible, but the problem if often that both platforms make it easier to write lazy code. there is nothing in each technology that prevents good applications being written in them and there are plenty of examples of that.
The biggest problem with wpf afaics, is that it's easy to create interfaces that are unperformant. There's nothing intuitive to the programmer as regards to what they need to do to make sure the application always run fast on the targeted hardware. It takes a programmer who knows alot of the ins and outs of the framework to write a good app using it. knowing when to use bitmapCache is one major example.