Accent on the second syllable.
Good:
Hardy.
Calm.
Good foragers.
Eggs tend to be large.
The French say the eggs last longer than others because the shells are thicker and the dark "paint" makes them more impervious. (After all, the breed originated in a port city for the purpose of provisioning sailing ships.)
Nothing looks better in your basket than those terracotta eggs, especially with a couple blue and green ones for contrast.
Bad:
They're expensive - an unfortunate result of being trendy.
There's a lot of dishonesty about them, again as a result of being fashionable. For example, some of the Cuckoos being sold are actually Barred Rocks.
Candling is difficult.
There's no American standard.
American lines aren't as good as European lines, and importation of new genetics is prohibitively difficult.
They're being selected for egg color, not production, so they don't lay as well as some breeds.
The ability to lay dark eggs waxes and wanes.
Interesting asides:
Sometimes the eggs are almost round.
Blacks and Black Coppers tend to lay the darkest eggs.
Wheatens are the most attractive birds. (That's just personal opinion, but I assure you that I have impeccably refined and sophisticated tastes.)