I use it on manual almost all the time. It's peculiar to my mild climate, but the auto setting is rarely right for me.
I don't like a draft blowing on my face, so I turn the vents up. The car will be cooled/heated, but I don't get a blast in my face.
The problem is, the AUTO setting runs the fan much too slowly for this to remain comfortable. I've tried, and in summer I have to set the temp about 6-8 degrees colder in AUTO than in manual mode (warmer setting, higher manual fan speed). Because of this, the coimpressor runs less on manual than AUTO, saving wear and gas.
On mild days (cool evenings on a warm day), I dont' need A/C at all. It's too noisy to leave the windows open, but the outside air temp is fine for me. So I manually set the fan at midspeed, no recirc, no A/C, and the temp all the way to max cold. This is basically creating a "silent vent wing" effect. Again, less wear and less gas.
This means I am running the blower more, perhaps wearing it out sooner. But a blower will be far cheaper to replace than an A/C compressor! I also like the fresh outside air (filtered, of course).
So, other than defog/defrost operations, I leave iton manual (mid fan, no A/C, no recirc), and set the temp as needed.