Yeah, just what I want. The "air" that the guy 10-rows up just exhaled/belched/farted out, blowing into my face... Yummy.
Any air vent blowing on me gets closed immediately after I sit down. If I'm hot, I'll take off my jacket.
Besides, the only planes I ever get on that are too warm are the one left by AA in DFW with the window shades open and the engines off. Get on one of those babies, and that little air vent won't help you one bit!