I have a few general questions about the bullet physics library.

Here is my current understanding in a nutshell:
btDiscreteDynamicsWorld - Simplest physics world, only handles rigid bodies, maybe it has better performance.
btSoftRigidDynamicsWorld - The only physics world that can work with large jello moulds
btContinuousDynamicsWorld - If you have really fast objects this will prevent them from prenetrating each other or flying through each other, but is otherwise like a btDiscreteDynamicsWorld.

Is my understanding of the btDiscreetDynamicsWorld, btContinuousDynamicsWorld and btSoftRigidDynamicsWorld classes in terms of functionality, purpose, and performance correct?

Why does the user manual recommend the btDiscreteDynamicsWorld class?

btSoftRigidDynamicsWorld appears to be the only world that can handle soft bodies, so what if we wanted Continuous Physics integration and Soft bodies?

How fast is fast enough to consider using a btContinuousDynamicsWorld, and what are the drawbacks of using one?

Edit: My Buddy Mako also posted this question on The Bullet forums: http://www.bulletphysics.org/Bullet/phpBB3/viewtopic.php?f=9&t=4863