A little over a year ago we decided to give Telerik's products a try as their products seemed to have finally matured to a point where we wouldn't need to worry about incompatibilities with other systems, mainly AJAX.NET. However, about 2 weeks after all the documents had been signed and development had actually been under way for about 6 weeks, Telerik decided to embed all of the web support files into their web DLL and broke the ASP.NET UI Control model. The Appearance attributes of all controls no longer work and the only way to control the appearance of controls is by overriding the attributes in CSS. There is very little documentation on the CSS and it requires a great deal of man-hours to complete. Additionally, if you need to make changes to the UI, you will most likely need to redo this all over again and waste even more man-hours.
About 8 weeks ago we decided to drop all use of Telerik products as their use was doubling the amount of time that it takes to develop a page. This meant that we needed to find a replacement. We looked at Component One and ComponentArt before deciding to go with ComponentArt. They have been very consistent with their products and we have used their products in the past when working with Community Server. The current controls appear to work just like the ones we used 4 years ago, and they are also 100% AJAX.NET compatible. The only control that they don't have is a CAPTCHA control, so we need to find a replacement for it.
We were really looking forward to a very long relationship with Telerik, and their Sitefinity product showed great promise, but they appear to be taking their own road (similar to what Borland did with it's C compiler) and have completely moved away from the direction that AJAX.NET is taking. Several of our projects were using the Telerik controls and it will take quite a significant effort to replace them with the ComponentArt controls. JawberDoo.com will be the first project that we convert, as it is the project that is closest to completion. We'll try to keep you up-to-date on the progress, but this is going to be a major effort and will take a great deal of time, so please bear with us.