I work with many diverse technologies, and don't possess the time to evaluate every single UI framework, so I'd appreciate hearing other coders' experiences with various frameworks.
The stuff I've worked with so far (and my perceptions):
I can really only offer WinForms experience, but I'll give a quick rundown:
I have used DevExpress fairly extensively on WinForms projects, and have been pleased with the quality of the components. I have actually been (generally) happy with the user API, though writing anything that extends their functionality is usually a substantial undertaking. Their visuals and feature set, however, are top-notch.
My last WinForms project was based on the Telerik WinForms suite, mostly because the developer who started the project (before I got there) came from an ASP.NET background, where the Telerik controls are evidently pretty popular and solid. This, unfortunately, is not true of the WinForms suite. The designers were extremely buggy (though more recent releases have helped this) and the feature set was...odd. I wouldn't recommend it.
I've also dabbled with Infragistics, mainly when we were searching for a suitable grid control at my last job (where we settled on DevExpress). I can't go into much detail, but a cursory examination felt like it was lacking both aesthetically and in the API department. They were a no-go for us pretty quickly.