Just read a very interesting article by Ben Forta. In the article, Ben suggests that CFCs should not be allowed to output anything to the screen. He suggests that CFCs should only be allowed to get/set/add/update/delete data and such, but that they should not be allowed to actually output anything to the screen. An interesting way of looking at things.
We have been dabbling with CFCs for a couple of months now, and have been trying to draw that line between what should be done by a CFC, and what should be done by simple CFML/custom tags/etc. Ben's article merits at least a look.