I have been using AJAX for some time now, though I do not code AJAX as you might think. I have been using the AJAX tools provided from Telerik to do so. Their tools have been straight-forward and easy to use much like any other web control. However, right now I am evaluating whether I should continue to use these tools with my most current project or move on to using Microsoft's AJAX tools. I have been looking through the "How To" videos at the following URL and I must say that this is a very interesting take on providing AJAX tools to developers. However, I believe that there is still too much for the developer to do upfront to always use these tools for your .Net AJAX needs.
As you can see from the following link to the Telerik web site, they are embracing the AJAX tools offered by Microsoft, but those tools do not completely replace the ease of use offered by the Telerik suite of AJAX tools.