telerik (excellent asp.net controls) entered WinForms controls market a bit late, but with an interesting innovation: their r.a.d. controls for WinForms are highly skineable (or themeable if you wish) and animatable in a WPFish way. Not that surprisingly they call their system telerik Presentation Framework (TPF). They have a nice hierarchy of elements including value storing in a bag instead to local fields. Another positive feature is that all the controls are built from small atomic elements so the control look is customized at this element level and doesn't need specific customization. Which is very good for you if you are extending TPF. telerik also provides a Theme Builder utility for your convenience, so you can build new themes or customize existing ones with few clicks here and there.
Even though TPF is pushing the limits of GDI(+) and telerik has done good job, it can't compete with WPF but it comes close as much GDI(+) allows. IOW it can be a good choice whether you are not willing to go with .net 3.0 for some reason (there are some reasons, among them the current lack of 3rd party market, customer doesn't want to install WPF, your application has to run on Windows 2000 or even worse, on Windows 98SE…) but you want or need similar UI. Perhaps the transition to telerik WPF stuff, which will eventually come someday, will be fairly easy due to similar architecture.
On the negative side, what is missing are heavyweight controls like grid, treelist and stuff like that to have a consistent looking application – you are forced to combine with non-TPF controls and loose consistency.