I've never owned carbon cages, and unless they're given to me, probably never will. They're awfully expensive for the very little weight savings. And I'm guessing by your question, probably a bit more fragile than traditional metal or composite (plastic) cages.
I love composite cages because they're both light and strong. Metal cages can be light but they get bent out of shape over time. Composite cages give you lightness with the added benefit of holding their shapes, and therefore your bottles, over time. I've used the same Trek/Bontrager cages for over two years and they're still working great.