Apple Insider is running a very interesting series of articles about Adobe Flash and its future. The first article in the Flash Wars series covered the history of Flash. The second part jumps in to all the hurdles and competitors Flash faces, from Microsoft to Apple.
ExtJS, in particular, can do about everything Flex can do, and do it without a browser plug-in. It also has APIs around the non-visual aspect of applications that put Flex to shame. Although ExtJS has licensing issues which I’ve griped about before, if you accept it at face value as a commercial product, it really is the best thing going right now for building RIAs.