My experience agrees with depressions: Contrary to what some people have said, air con only makes a tiny difference to fuel economy in practice. The dehumidifying effect is well worth the extra couple of mpgs - not only does it keep all the windows permanently clear, but it also feels a lot less "sticky" on a cold damp day. So I don't use ECON at all these days.
Every week or two in the winter, I make sure the air con compressor gets a proper blast by waiting until the interior is nice and warm, then switching to RECIRC and LO for five minutes or so.
Last winter, when the car was still new to me and I was using ECON, I would get a musty smell whenever I did switch the air con on. But this winter that hasn't happened at all.