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.
That's great. Thanks for the response. I've always liked Jimmys surfboards and they are trully bombproof. Bad news are that i'll need to find the money to get both of them and hide it from my wife at the same time.
I have the 5'11 KWAD and love it... never rode the chamber but have ridden tons of thrusters of similar rounded squash type of shape and Ken's description is spot on. My buddy went from chamber to kwad and really likes the kwad. I would also look at the conditions you will be riding it in. I think the kwad is a great all around board for most surf conditions and the chamber would be great for larger/hollow surf. the kwad has done well in larger/hollow surf for me so it can handle most anything I woudl go out in. upwind performance on the kwad is awesome. kwad does really well in light wind. not sure how it compares to chamber. I am 185lbs and the 5'11 seems like a good size for me. Woudl not want it much smaller since I don't like riding so powered up and woudl rather surf the wave a bit more. jsut my preference. Cannot go wrong with either one... JL makes great boards.