The HPG Original Kit Bag was one of those pieces of gear I resisted for quite a while only because I was so tired of hearing people rant and rave about them and it felt like a lot of bandwagon BS. I gave in and bought one and am a pretty big fan. I have carried guns as small as a S&W 642 Airweight .38 Spl, Glock 19, on up to a 4" S&W 629 .44 Magnum. It carries these guns well and offers fairly quick access.
I have used mine for carrying a gun while hunting, camping, backpacking, trail use and also in town while running stairs. I put my phone in there and have the headphone wire sticking out of it so it looks like I'm using it as a way to carry my phone, etc. (and a Glock).
Pros:
1. Compatibility with wearing a backpack. The gun isn't on your waistline interfering with the pack belt and also the gun isn't attached to your pack hip belt so when you take your pack off, you're still carrying your gun. This is handy when filtering water, dropping a deuce or milling around camp.
2. Depending on the color, it doesn't scream "gun" in those instances where it might be an issue. My dad just got the OG kit bag in black and I kind of like it. I wish I'd maybe have got foliage green instead of coyote, but it's not a huge deal.
3. Allows you to carry more gear than just a gun. Fire kit, emergency gear, etc. Depending on the size of the gun you can also carry binos in there if using it as a bino harness. I did this this season and carried 10x42s and a snubby with no issues.
Cons:
1. You can overload them if you aren't careful. It ain't a day pack!
2. It can take a little getting used to carrying stuff on your chest and also ironing out the draw stroke of presenting the pistol.
Just another option out there that works for some better than others depending on their application.