They're great if you travel a lot and don't like to carry a backpack or large bag everywhere. Phone, charger, usb cables, travel doc's, tickets, extra cash (divided up in several pockets), can make it really useful.
Unless your firearm is thin, like perhaps an M&P Shield, they aren't that great.
If you stuff them too full of essentials you'll end up looking like a shoplifter and get stopped by store security often