I'm currently using GraniteDS with Seam 2.1.
It works wonderfully well. They mirror the Seam contexts on the client (including conversation contexts) and the objects are synced automatically (in fact they modeled the initial iteration of the framework on Seam). Object uniqueness is guaranteed on the client contexts (per context). Additionally they have their own Tide framework that does an excellent job at replacing the need for Cairngorm (or any other MVC client framework) and they have excellent documentation online. Also the transfer protocol is AMF - not to mention that they support data push. Having migrated from BlazeDS, Cairngorm, Spring to GraniteDS, Tide, Seam (with Hibernate) I would highly recommend it. Not only do I feel that the combination is perfectly mature but it also provides advantages in ease of use / features that rival those of LCDS only free.