I'm using Appcelerator on a current project; it offers an easy way to get the sizzle of AJAX in your web application. The Web Expression Language takes a while to grok but you're rewarded with a very easy and succinct mechanism for connecting events to actions that eliminates the vast majority of JavaScript that's required of other frameworks. Another aspect I particularly like is embedding the Web Expression Language within the page layout; it makes it very easy to fix problems and add new functionality.