I jumped on the ASP.NET AJAX bandwagon when the Release Candidate came out. I've heard a lot about it ever since it was called 'ATLAS', but I had never played around with it until recently.
The Official 1.0 Release version came out on January 23, 2005 and the Full Source Code became available today.
Along with the framework, you can download the AJAX Control Toolkit. You can demonstrate the controls from this toolkit on their test website.
I am quite impressed at the easy, speed, stability, and breadth of the framework and controls. If you haven't played around with this yet, then you are missing out on some excitement!