Let me first apologize for all of the trouble and negative experience you're having with the new release of RadControls for ASP.NET AJAX. Obviously, we've put a lot of work in to making the upgrades as seamless as possible, but there are still many things we need to do better. Your feedback will definitely help us focus our efforts. In the mean time, let me address a few of your concerns.
The migration process from classic controls to the new ASP.NET AJAX controls should definitely be a planned effort that you set some time aside for (when and if you choose to do it for existing projects). The new controls have been completely re-architechted on the ASP.NET AJAX framework to deliver the most optimized rendering and client-side performance possible.
We always do our best to give prior notice on important changes and to mark properties/methods with obsolete/deprecated where needed. We also maintain a section in the documentation for changes and backwards compatibility and upgrading from previous version (For instance: http://www.telerik.com/help/aspnet-ajax/grdmigrationtoprometheus.html
Over the years customers have liked our aggressive release schedule with new features and improvements being added every month. We always strive for excellent quality but unfortunately some problems slip up in our QA process - that is why we fix them immediately upon anyone contacting us. We also put sticky threads in the forums with known issues but in most cases we just fix them ASAP.
We are always available for assistance while you upgrade to a new version. Most of the times it is really smooth but if you hit anything unexpected please send us a support ticket and we will address it right away.
I want to thank you and all other customers for providing continuous feedback on our releases. We appreciate your input! We are doing our best to improve with every release and your help is really great.
All the best,
the Telerik team