There are many RIA frameworks popping up (flex, extjs, gwt to name very few).
If my server is written in Java, why not use java web start?
The benefits I see: I get to program in a language that is easy to debug (compared to javascript), I can use the same code in server and client side (validations, model objects). With SWT I get nice looking GUI.
Even if it means requiring a plugin in the browser (or using a cli tool), if my application is an enterprise application, does it matter?
The only contra argument is that in a lot of cases it's impossible to force the clients of your rich client app to have Java runtime installed on their machines. Everything else is a matter of taste, already existing knowledge and experience.
For webstart you need Java installed on the client. For Flex Flash. For Silverlight .Net. For other RIA platforms a JavaScript enabled browser. It is a lot of product politic in that question.
And Java is very intenting, because "all seems to be a nail". Of course one of the great advantages of Java: same language, same knowledge, same code on every platform or environment (server/client/middleware).
For enterprise internal use, no it does'nt matter at all. If your biz is on the internet the decison is much more complicated (or even you have to offer the same functionality in diferent platforms).
Yes I think it does matter, seeing that some big companies still have IE6 as their standard browser and as a requirement for web based applications. And those won't allow you to install any third party plugins into their dinosaur browser.
And you don't make a difference between RIA technologies that use existing browser technologies like JavaScript, CSS and HTML (including GWT which just generates client side JavaScript and ExtJS which is just a JavaScript Component library) and the ones that require an additional runtime environment (Flash, Flex or client side ).
Of course some things are easier to accomplish with proprietary or third party solutions (or you just have more knowledge in one of these platforms), but I still think that there is a huge potential in technologies that are standardized and already implemented in every modern browser (the differences between the browsers are slowly but steadily getting better, too).
If you ship it as a 3rd party application you can define a JRE as a System requirement, anyway but then instead of JavaWeb Start ship it as a normal Java Application as well.
I think that until now the perfect client technology simply does not exist. We were excited about ajax and then there is another "hot" technology like Adobe Air. What is the difference between these techs and the "old-fashion" client-server apps? My friends we are back to the early 90s! The AIR for example needs a runtime installed on the client machine, it has access to local system resources and communicates with the server using a binary protocol. On the other hand we are developing "RIA" applications (and very good ones :) using pure java swing clients communicating with the server through the Spring's binary remoting protocol. And i must say, in such a development life-cycle Java-Webstart is the winner. Especially in large-scale projects (and i mean laaaarge-scale, not just e-shops) in the java field i haven't found something more productive and maintainable (ah! maintainence... another big story) than this. IMO if your users can live with a nice look-and-feel and you don't have to adopt another technology just because it is young and sexy use webstart. Put more work in your business model. After all enterprise apps is about business, not animations. And money is where the business is ;)