I have the LG one from Nashbar. Trouble is, my head sweats mightily under it. I just need the ventilation more than the helmet cover. Even in January.
My Specialized helmet has a big opening for air on the front of the helmet (see that black space on the bottom of the helmet at the front), sorta like this:
I've found it's not to bad if I tuck the helmet cover just above the opening, and leave some space open on the back for air to flow out of the helmet.