So far it is working very well. I haven't checked the integration recently, but I had to do a little bit of framework stuff to make the Box2D objects integrate well with cocos2d objects, but now it is running smoothly and it's easy to add new objects!

If you want to sync the cocos2d objects based on the box2d objects, you need to query the objects, and then update the cocos2d properties (box2dtest.m shows how to do it).

If you want to update the box2d objects based on the cocos2d objects, you might want to create a Box2dSprite that overrides setPosition and setRotation.

center of gravity can be changed by using [sprite setAnchorPoint:ccp(x,y)];

Box2D was definitely a great addition to cocos2d, although you have to work through little quirks of using C++ with Objective-C. The API for Box2D is definitely more mature and detailed and more attuned to the needs of the programmer, I would say.

Now, on their own, both cocos2d Sprites/AtlasSprites and Box2d are awesome APIs. But there is always this need for bridging the two and make them work together. It would be nice to see Box2D well integrated with Sprites.

For example, if I have a set of physical bodies with arbitrary shapes attached to it, the following work is needed:
- define Sprite textures that are identical in dimensions to the shapes.
- move/rotate the sprites as and when the bodies move/rotate.
- (more specific) keep track of the center of gravity of a body so that the Sprite can be positioned/rotated around the center of gravity, and not just around the texture image's center.

If these correlations and others can be offloaded a bit, that will lead to a closer integration of cocos2d and Box2d.