I switched from Zen back to Firefox because of bugs and forced features like window sync which many users including myself don't want. I really miss having the entire UI on the side, I really don't care about Zen's other features.
They made a dumb new feature and rolled it out without thinking of how users would respond. Essentially it syncs all of the same tabs to multiple browser instances. It can't be turned off, but it's stupid that it was released in the first place.