I've ridden all three. Compared to the Chamber, the newer Shack has been thinned out and the profile has been refined. Jimmy felt that the tail on the Chamber was a little abrupt and so he blended the curve a little more on the Shack resulting in a smoother carve. The thinner board makes for more flex and response. It's still a very tight carving board with true thruster characteristics.
The Kwad, however is a different animal. Its tail releases a lot easier for controlled slide. It has a looser, skatey feel where the shack is a tighter turning clean carve. Both are great, but it depends on how you like to ride.
This link is a side by side comparison of the Kwad and Chamber from before the Shack was released. The Shack is similar to the Chamber--but just a little better.http://e2.ma/webview/2nsd/b3dd8dc699736622c3b2fb80af072e81