Open source, having full control is a big benefit. Some game companies choose Bullet over Ageia and Havok because of this, it can give competitive advantage when developing novel physics techniques. The entire Bullet collision detection fits and runs on one or more SPUs (or multicore on XBox 360), including streaming of compressed static triangle meshes (without SPU 256k size arbitrary limitations), convex polyhedra etc.
Another aspect is that Bullet has been ported to XNA / C#, also known as Managed BulletX
