Checking out a post by Charles over at Flex888, I was prepared to laugh again at ZK's historically clunky JS performance and experience.
Instead it's grown up.
Awkward bumps in hiding, showing, and simply using have been replaced with graceful transitions typically seen in raw usage of libraries like JQuery/MooTools and the like.
Having battled Richfaces lately on a product I'm almost at the point to ditch it for something streamlined and reliable.
I used to think of ZK as a poor alternative to Echo, but these days I'm pretty convinced it's worth a second glance if you're looking for one platform to do everything.