I have really fine hair that dries really fast, so I've almost always air dried.
The only downside for me is that if I need volume, I then have to use hot rollers, which kind of offsets any benefit from not using the blow dryer. On the other hand, I don't need volume on a day to day basis.
I find that when i air dry completely, my hair is flatter. So I do a couple of minutes of blowdrying on the warm setting and dry about 50%, letting the rest of the hair air dry. I've heard that if the blowdryer is too hot for your hand, then it's hot enough to damage your hair.
Kelowna, British Columbia, Canada
I air dry my hair almost all the time. My hair is long, thick, coarse and wavy, and extremely healthy. I also rarely use hot tools and use just a little curl cream/detangler after washing it, which I do usually only 3 times a week.
Maitland, New South Wales, Australia
My hair takes days to air dry, even if its been brushed and left out (and you're not supposed to brush your hair when its wet anyway). Say I'll wash it on a Monday morning or will still only be half dry by midday Tuesday.
My hair grew much longer when I didn't blow dry. Then I got a crazy busy retail job and was blowing my hair every week. When I wasn't blow drying I was using flat/steam irons. I just cut my hair to chin length because once your hair is damaged you spend more money trying to repair years of abuse.