D Trent Childswrote a review of Park Tool DAG-2.2 Derailleur Hanger Alignment Gauge on April 26, 2018
Familiarity: I've put it through the wringer
If you find yourself bending derailleur hangers more than once in a lifetime then I'd recommend snagging one of these. They get you derailleur tweaked back into shape with ease. Much easier than buying a new hanger and changing it every time it gets dinged.