You buy a face mask for protection!!! Anything else is just icing.
After taking a puck to the grill this winter, it was time to put a cage on. So I bought a new Bauer 9500 helmet with cage and dropped the cage for the half shield/mask. I wanted the unobstructed view of the shield but have full face protection.
The lower cage allows more airflow than a full shield and the half upper shield gives you less obstructions to your vision. Its not perfect, but its the best of both worlds and I get to keep my teeth. Maybe if I made what Duncan Keith makes to play hockey (or any money at all to play hockey) I'd be less concerned.
Anyway... the shield mainly steams up when you hit the bench and the air flow stops from skating around. Its a full mask, so its a pain to lift up to get water and wipe sweat off of, etc... but again... its FOR PROTECTION!!!!