I used my BBO this weekend into the mid 20s. I left the head end loose and had no condensation issues. I only close the head end when there is a steady wind to draw the condensation from the fabric.
It certainly warms things up!
On a down note, my BBO stuff sack disappeared over night. I think it might have blown into the river I camped near. BUMMER!
"Never tell people how to do things. Tell them what to do and they will surprise you with their ingenuity." Gen. George S. Patton