Cons:
1. Heavy. Somewhat. There are some that weigh only a little more than a normal helmet: Casco Viper and MET Parachute. Unfortunately only sold in Europe, but thanks to the internets, can be bought here: chainreactioncycles.com.
2. Hot. Not necessarily. The Specialized Deviant is reportedly as cool as the Giro Xen due to the 21 vents. Of course the insulating properties can also be a pro in the winter.
3. Reduced vision. MTB'ers report most full face helmets not to be an issue (see reviews at mtbr.com), but of course something that protects your face probably does get in the way more.
4. Clownish. Yes, you may be ridiculed for wearing one, but mainly from the other cyclists you hang around. You shouldn't wear one if you don't take peer pressure well or you have a fragile ego.

Pros:
1. Obviously, better protection for the face as well as back of head and neck.
2. Keeps you warm during cold-weather riding.
3. Motorists may give you more respect on the road since you look more like a scooter/motorcycle.