I use it here as well. There's a bit of lag when I use it over wifi (air display) and much less when connected via cord. Not as good as an actual external monitor but it does work very well for my needs and it doesn't clutter up the desk like an external monitor. FWIW, I like air display better than duet as you can sync them over wifi, which is convenient sometimes.
Yes, I use it often. The main issue, in my experience, is that when Duet display is set to a good quality, it just sucks up the MacBooks battery. You won't get many hours of use that way. So, it works best when the MB is powered.
I originally used AirDisplay, but it gave me so many issues I replaced it with Duet.