Won't help with the sulfur dioxide or ozone (from smog), but filtering out particulates ought to help. At least on long time duration trips, every time you open a door....
True on the first. On the second, I would bet that the entire air supply in the car could be sucked through the system in under 3 minutes once the door is closed.
Too expensive for me, but it would be really nice to have. I use only HEPA vacuum cleaners because by definition they suck up a lot of allergenic dust.