ASP.NET AJAX is great right? Just throw a script manager and update panel right down on the page and wahla… Not so fast. Although ASP.NET’s baked in ajax controls are very useful, you have to be careful that your ajaxed-up code works well on your production system. I recommend that if you are using ASP.NET AJAX, to do a quick POC on your production box to make sure everything works as you plan. You can especially run into trouble with SharePoint! Also, before you dive into this stuff, check out the link below… It explains exactly how ASP.NET AJAX wipes the page and how you can use JSON specific calls to only update the page where it is necessary.
This website will explain exactly why UpdatePanel’s are dangerous: