I am wroking on Telerik controls from 1.6 yrs in my company so would like to say something on it.
Telerik controls are third party vendor controls which are designed and created by Telerik organization. These controls are mainly designed on behalf of ASP.net controls though we have a lot of controls in asp.net but still today many company are using these Telerik controls because it provide good UI and look and feel approach. It will help to design high quality line of business application.
All the telerik controls has good look and feel which attract more user to use your application. The telerik controls not only made for asp.net web application but it also have incredible mobile UI as well as asp.net mvc controls, Silverlight controls and mainly asp.net ajax controls. It provides a flexible and outstanding feature to smart phones.
You no need to add any extra css on top of the controls to decorate them as telerik will do for you. It provides a lot of Demos and example program for each and every concept on its website which helps developer to refer those before start the development process with telerik. Telerik controls have good support team as well which will help when you actually indeed them.
The main problem with these controls is they all are autopostback as per as asp.net application are concern. All the telerik controls has this property by default true. Even when we are working with nested grid controls while expanding and collapsing the grid it will autopostback to server which degrades the page performance. But you can solve this issue by making autopostback property to false.
Apart from this we have some other small issues as well. Some of the telerik controls are not work properly as expected. Recently I faced certain issues when I was working with telerik controls from asp.net user controls.
But despite of the cons what I have mentioned over here, Telerik are excellent in rest of the areas. Nothing is 100 % excellent and perfect in real world so obviously not telerik too. But we need to emphasis more on its positive aspects rather than negative.
To know more about telerik controls and to see its demos please visit to: http://www.telerik.com