Well, ASPX Forms was looked like cool several years ago only because of satellite technologies like C# programming language and Visual Studio IDE.For now ASPX Forms has no real pluses but a lot of minuses.
StateView made the page very huge, Client-side ID’s of controls.
AJAX that is not recommended to use even by Microsoft. (S.Baydachnyi said that on the Microsoft Days Conference). If you need real web or web 2.0 you better use MVC.
It discipline you to program real web without those terrible Forms.
You have easy way to port you solution to our MVC framework like RoR, Django, etc. In case of Rich web app it would be better to use something like Flex or Silverlight.
I mean back-end interface AKA admin area.
Even Dynamic Data tech. (based on ASPX Forms) can provide only very basic things. Unfortunately, there is a lot of legacy ASPX Forms code needs to support.