I would emphatically recommend DevExpress for a Winforms control suite, and emphatically recommend against using the Telerik suite.
I've had a lot of success with the DevExpress suite, and while their API is a bit complex, it's still quite manageable. And the controls are rock solid.
Telerik makes fantastic ASP.NET controls and utterly terrible Winforms controls. Telerik attempted to bring the WPF binding and appearance API into Winforms, which created massive complexity (as now you're dealing both with the Winforms and WPF API's) and unreliable behavior. While they have improved substantially in more recent versions (for instance, the MultiSelect property on the grid now finally allows you to select more than one row and the designers now don't cause a complete loss of all designer-generated code every time they're used), but there's still a long way to go before I could recommend them. I've just finished deploying a fairly simple (from a UI perspective) project that utilized the Telerik Winforms controls, and I can honestly say that we spent substantially more time chasing down bugs and inexplicable behavior in their controls than we did writing the rest of the application.
