Adobe Flex 2.0 simplifies development and deployment of rich Internet applications by providing a large number of prebuilt, browser-agnostic components for interface design, data display and manipulation, and audio and video output. In addition, the ActionScript language provides procedural control of the application when declarative MXML won't do. Finally, Adobe Flex applications are, at their core, Flash applications, and therefore supported on the vast majority of browsers.
Adobe has taken a different tack from other vendors regarding browser support, relying on the ubiquity of the Flash player to power its Ajax offering on the client. By making the core SDK freely available, it stays aligned with competing vendors (Google, Yahoo, Microsoft) that provide free toolsets as well as the open-source community (Dojo, Rico, Zimbra's Kabuki). In addition, Flash is perceived as being more secure than client-side scripting. However, there's no getting around the fact that using Flex locks you into Flash player, a constraint some may find unpalatable.