I made a mistake last year getting on someone's azz about using one. I cut by eye and have for years, but we had a hired hand last year that could never get the "eye" for it. I've read more than one post where people have said they work great. If we hire another guy who can't get the hang of it, I'll buy a Mingo marker, it would pay for itself by not taking the time to clean up 1 guy out of fives work.
We cut to order, I've had request for wood from 10' to 30", it is hard for some guys to change up, they get to used to 16" wood.