We are using the Telerik controls for WPF. Initially we started out by rolling our own grids, combos, trees, etc, with custom styles and templates, but when it came time to polish everything up, we needed all the added functionality that Telerik provides. We are disapointed in some of their controls, like the carousel, but we are working with them to improve the control. We often find that the support they give (usually with complete coding samples) offsets the cost of the package. They follow Microsoft's naming conventions pretty well, so the learning curve is not too steep.
We have tried ComponentOne, but don't like those.
