If you're looking for Mac OSX compositing programs, you've really only got 3:

After Effects
Combustion
Shake

I've used AFX regularly, and I like it because of the simple interface. Not a lot of clutter to get in the way. As has been mentioned, there is a bit of a learning curve, but it's worth it. It's especially nice to use, because a lot of terminology is the same as Photoshop's. AFX does a particularly nice job of handling alpha channels.

I have used Combustion a bit, and while I like the interface better than AFX's, I don't know it well enough to be really productive. The interface is quite streamlined and is pretty to get used to once you start working. I actually stopped learning Combustion a while ago, because we use AFX at the school I teach at. Combustion has incredible integration with 3ds max, but that's not terribly useful here. There is a MEL script that will move camera and object animation data to Combustion.

https://www.fido.se/mtoc/index.html

Not sure it still works or if it even works with OSX, but it's worth a shot (I haven't tried it in a while).

@Nem,

Both Combustion and AFX have 3D workspaces. You don't really have 3D objects to manipulate, but you can move the 2D layers in all three dimensions.

To add depth of field in AFX (for instance), you could set up an expression that adjusts the amount of blur on a layer depending on its distance from the camera. For 3D fog, you could try a particle system on a layer with some sort of blending mode.

Just remembered another program: Commotion Pro. While it's not marketed as a compositing program (more on the video painting and effects end), it shares a lot of features with AFX. It actually handles alpha channels and chromakeying a lot better than AFX, but it can be on the slow side sometimes. I usually use Commotion Pro when I have to clean up alpha channels or do a lot cleanup on individual frames.