...over a regular metal post (vibration absorption, etc.), or are they just cosmetically pleasing?
The latter I'd say.
My experience has been that they are inferior to an item made solely of either one of the materials.
As the wrap is so thin, it often cracks easily.
the only carbon/alu combination thing i've used recently that's been any good (apart from a frame) are shimano's rs80 wheels which do offer a very good mix of stiffness and vibration damping. but only because the aluminium is thin enough for the carbon to have an effect on the material properties of the rim.
They look better for sure but thats the only benefit I can see in having a wrapped seatpost or stem of whatever. I would say its better to make a good quality product out of one material than to try and make a nice looking product out of two.
That's decided then. Cheers everyone.