I own a micron gauge.

My comercial friend says freezers or chillers which are way below freezing are much more critical of moisture, hence 200 microns is what he shoots for. I imagine with all the rubber hoses, o-rings etc you'd be hard pressed to get a leak-free automotive system below 400 microns after it sits for a half hour or so. I've flushed systems and then pulled them down, and you can watch the vacuum decrease as volitles boil off. Very cool stuff.
I find the gauge good to test the vacuum pump. Still PO at my robinair expensive pump which is rated at 4 microns and one day does 400 and the next 150. My comercial friend retires his pumps when they won't pull 200.
Harbor-freight had a 3 CFM, 2 stage 25 micron pump on sale this past week for $120.
Oh- and the digital micron gauge is helpful in leak checking your manifold/hoses etc.

Michael