AjaxControlTookKit is a very nice addition of Asp.net 2.0 Ajax Extension. Recently I had very nice experience using asp.net 2.0 Ajax in few of my projects. "update panel" is very use full control as far as easy Ajax is concern. I haven't explored the whole area of client side scripting, but so far the experience with the server side controls are pretty much good and satisfactory.
Problem with Calender
In one of my project, I had to use the Calendar Extender of Ajax. Well in the page level the Calender Extender worked fine. but when i put the extender in one of my usercontrol. after the page render in browser the calender popup's position is pretty strange. No deterministic behavior was found in popup. when I click on the button of the calender the popup window appeared way up where it should actually show.
I went on trying few things to fix the thing and succeed on the setting position to relative. I just put the position behavior of the parent div of the Extender to relative.
worked like cramp
If you set the css property of the calender extender suddenly the calender loose all its default behaviour and color skim. And no detail guide line is found in the ajax.asp.net site for customizing the style of the calender control.
Since ajaxControlToolKit is under development we hope this issues will be fixed in the actual release product.