There are a lot of ways to break a rigidbody dynamics system, and Bullet is no exception. It will require a lot of tweaking to get things right, both for the dynamics system writer and for the user.
Unfortunately small object sizes, lack of friction and damping, too much collision detail, large mass ratios, and many more issues can break the simulation.
First thing will be to get Bullet fully integrated in Blender and released for all platforms, Windows, Linux, FreeBSD etc. I don't expect small jenga stacks on a billardtable a realistic goal in the next half year
As a small example of commercial engine tweaking, did you read topic
http://www.continuousphysics.com/Bullet ... c.php?t=54 ,
Chris Butchers posting about Halo 2 and large mass ratios with Havok ?