I think overall the documentation is fair. I definitely experienced a slowness in development when I started using some of the Telerik controls for the first time. There is a learning curve. I've used them for a few years now so I have a much better idea of how to approach issues. Having said that I do wait 1-2 days for reply's to issues or sticky points in the code where things are not working out as I would think they would. I do feel that the examples Telerik gives are most often the most basic of scenarios. When coding in the "real" world you know that these scenarios are not the norm.
I would rate Telerik overall with support and documentation around a 7 out of 10. They have a lot of controls and its not easy to keep it up to date when you consider how aggressively they roll out new products.
The controls themselves are pretty good. Not sure when ASP.NET 5 and Windows 10 are out however if I don't move to another platform altogether.