What do you know about ZK?

A week ago, for the first time I read about ZK http://www.zkoss.org

It exists since 2005, so I thought to myself (and answered) how come I didn't bumped into it till now? It looks like ZK has a lot of use http://www.zkoss.org/whosusingzk/ ,Seam integration http://www.theserverside.com/news/thread.tss?thread_id=46501 and a new release 3.5.1

Did anyone of you guys had any experience with ZK?