We use Telerik a lot, togheter with the standard set of controls.
Pros
- Prevents you from reinventing the wheel. For example, RadScheduleView and their charts are powerful, if you need them.
- Tons of controls, for almost any need
- Look and feel-consistency
- The huge library of demos and examples provide a solid base when you want to rapidly do or test something
- Extensive documentation
- Great support from the staff and community on their forums
- Widely used - almost every issue encountered has an answer
- Quite good MVVM-support
Cons
- Some controls are sub par performance wise, but it has gotten better over the last few years
- You probably wont use most controls
- Your application tends to get rather large (even with optimizations)
- The learning curve for some of the most useful controls can be steep, depending on the experience you have
- Customization of some of the controls is cumbersome
- Some controls don't offer much above the standard ones
My tip is to find out what you want to do, and check if they have a control that seems to make the cut - and try it!