This was a question posed to me by a friend several months ago as I was first getting up to speed on the Adobe Flex platform and expressing some enthusiasm. At the time no immediate answer came to mind. I had just finished up some AJAX work and knew that AJAX is a powerful approach to building rich user-experiences and I scarcely knew the Flex platform had to offer. Today I ran across a post from EffectiveUI that really lays out the advantages of each technology and when it makes sense to use each. Well Done! http://www.effectiveui.com/blogs/?p=133
Also, I’ve come across several apps that demonstrate the kind of functionality would be either impossible or prohibitively expensive to develop in AJAX/HTML:
* Picnik-an online image editing service that integrates with Flickr, Picasa, and others
* Papervision3D–Open source 3D engine for the Flash player
* Google Finance-The stock chart here is built on the flash platform