One of the other teams here at the office spiked out the use of ZK and was kind enough to give me a demo. It looks like a wonderful tool for building a rich user interface, and I'm thinking that we'll probably use it on the new application we're building. (Sprint zero starts next week.)
I had begun advocating Flex as a way to build a "real" UI. It seems like a good fit for this organization, since SEO is much less of a concern for us than it would be for someone publishing open-web applications.
Flex does have some drawbacks, though. The main problem is that we don't have expertise in Flex's XML format or ActionScript in-house. There are a lot of very good developers here, but it still takes time to learn a new tool.
Expect to hear more on this as the team takes a deep dive into what ZK can do.
Follow-up, January 2012:
I was out of the loop on using ZK for a long time. Now that I'm the team stuck with the codebase built on it, I can render a stronger opinion: ZK sucks. it is nearly impossible to test.