ATLAS is a really nice piece of technology, allowing you to AJAX enable an ASP.Net page. The recently released ATLAS Control Toolkit provides a suite of ATLAS controls that are useful out of the box, and provide a good set of samples for developing your own.
Of course, the problem with using any third party control is that you may have trouble when the control doesn't work in quite the way you expect.
I've been having a problem with the Popup Control Extender recently. It's a control to let you use one control on an ASP.Net page to toggle the visibility of another - and being part of the ATLAS Control Toolkit, it's fully ATLAS enabled and so renders the behavior on the client.
The problem was that the designers of the toolkit didn't consider the case when the extender was used in a composite control that was repeated on a page. The toolkit renders markup on the page for behavioral properties exposed from the extender, but wasn't catering for when the extender was itself repeated as part of a composite control.
Fortunately, the source code for the Popup Control Extender comes with the ATLAS Control Toolkit itself, so I was able to fix the fault myself - score one for open / shared source!
I posted the full fix in the ASP.NET Forums - hopefully others will find it useful. I'm just made up to be able to contribute something back for a change!