Dietrich J Kappe explores ZK in details and has an interview with the ZK creator Tom Yeh addressing the ZK status and future plan.
Tom also discussed the differences on
Echo2: Echo2 assumes UI designers are Swing programmers, while ZK assumes many of them are not programmers.
GWT: GWT is a client-side solution and quite good for developing Ajax components. On the other hand, it is never a good idea to replicate the business logic to the client, which eventually brings us back to the maintenance headache of fat clients.