I've been carrying this bag for some years now, and it really was a game changer for organization and preparedness. Having everything I need, and many things that I may need, all together in one place ready when I leave the door is something I can't imagine living without now. No need to search for my keys and other necessities, forgetting things I should have with me and having to go back, instead having everything ready to go whenever the need arises. My shit in one sock, as they say. It was especially convenient when I had two vehicles and could just grab the bag and never worry about being without. This bag goes with me when I leave the house and rides next to me in the passenger seat (when I'm alone), everything I need right there in reach.
The bag I chose was an Oakley sling bag. Its velcro backing on the inside allowed me to integrate organization accessories from other companies. I have everything I could think of needing in this thing. Batteries/chargers/cables, bit set, flashlights, knives, pen/pencil/sharpie, lighter, tape, headlamp, mini tools, first aid kit, gloves; I even found some thumb actuated sea salt/pepper grinders, and a few packs of hot sauce of course. Right on the outside there's a spot for a velcro patch where I was able to integrate a cell phone pouch to have it right at hand. I also have a few other things attached to the outside like a folder, Swisstool (in pouch), another flashlight, and a strap cutter just to have them handy in a pinch. Then in the dump portion is still room for a snack pack, camera, and anything else I might find the need to throw in loose.
One vital feature of this bag, and probably the one that I've found to be the absolute most convenient is the way of storing my house key. There is a section on the strap with smaller straps where I slipped a kubaton style keychain on a key-bak system. This means when I leave and return to the house my key is right there on my right shoulder. I just reach up and grab it, extend it with the key-bak cable, lock/unlock the door, and let it return to its position. There is rarely a need to ever remove it from the strap. I haven't had to search for, nor even fumble with my house key in years.
I must add that as convenient as this carry bag system was for me, it was just as much fun to put it all together.