I haven't used one as I haven't reloaded bottlenecked cartridges yet but I get the impression they make life easier. I plan to get one when I start loading 30-06.
I have case gauges for all of my calibers except wildcats. If could have one for the wildcats, I'd have one for them as well. Easier to use than your barrel or measuring. You can make a quick visual inspection of your brass to determine if it's resized correctly and if the brass needs to be trimmed.
A decent caliper set and a shoulder datum bump adapter is all a man needs imo. You can check proper sizing or remove the gauge and check oal for trimming.