I changed from Havok Physics because the license for using Havok Physics was far too restricting, whereas this Bullet Physics license is probably the most liberal license there is. Another good selling point is that its free.
I decided to move to Irrlicht after having too many problems with Ogre, and so far I've been doing fine. Its very easy to set up and use straight of the bat. Its opensource and has a very nice and simple license.