I've ben working on an extjs app for the past few years. My experience is slightly different. I've found development actually speeded up. The extjs components are pretty mature, which is probably part of the reason. Benefits so far are the ones listed in the article, and also a structurally lower cost to supporting different browsers (even ie6), and a much easier time validating the security architecture, because of the reduced surface between client and server. Oh, and our users are very enthusiastic about the improved usability of our apps. They keep asking when we'll move over our older apps to the new architecture.