Looking at some of the many interactive web GUIs available now, both proprietary and open source, it looks as if they are reaching the point where they can pretty much match Java or Native client/server applications for functionality if not quite speed.
The huge gain is in developer productivity - with widgets and dhtml/rpc it is possible to totally seperate the presentation and application code, reduce the complexity of dealing with the client at all (the better ones will work correctly in any modern browser without downloads or plugins) as all the client work is done by the toolkit and browser.
nat friedman mentions this in his blog here and here
Some other web ui toolkits, that unlike Dojo actually exist with examples/documentation/etc, are :
