Macromedia's Macromedia Flex Web user interface builder (RIA Rich Internet Application builder ) gets very good marks, especially for fast development of rich client interfaces; nearly standalone operation, which eliminates the slow Web refresh cycle; ability to run in any browser and on all versions of Windows, Mac, UNIX, and Linux; and ability to run on Macintosh and Windows in Dreamweaver. However, back-end connections are still under development, and a Flex developer requires a fast PC or Mac with 512MB of memory. Application startup times vary widely, and coverage for personal digital assistants (PDAs), mobile devices, and special devices is spotty. Flex runs as a presentation server in multiple Java server platforms, including Tomcat 4 and 5, Macromedia JRun 4, IBM WebSphere, and BEA WebLogic Server, among others. Because the Hypertext Markup Language (HTML)-based interface is becoming outdated, and due to demand for rich media, such tools as Flex and rivals AltioLive, Droplets, DreamFactory, Isomorphics, Laszlo, and Nexaweb are presentation servers that use some slight of hand to download a plug-in to the client, which is a base for providing more standalone operations. Flash is used to display components on the client, and CSS is used for formatting. Also described are data binding; customizability and extensibility with eXtensible Markup Language (XML); a soon-to-be-available Flex to Dreamweaver; and performance of the Flex server during testing.