This is the third year that I will be carrying one. I've gone through several different bags, the one I have now is a good balance of size and portability. I got it at a local military surplus store so I can't recommend looking for this exact bag, but I'll discuss why I like this one.
I'll also be deconstructing my bag and rebuilding it with some new ideas that I got from looking at the survival kit instructables.
Aside from having the resources in this bag at my disposal at all times, it also helps me to not forget things because they're all in one place. I usually don't loose my keys, because they go on my Possibles. I don't loose my phone because it's in my Possibles. I have on occasion misplaced my Possibles, but it's kinda big and not easy to miss.
Step 1: Don't Call It A Man Bag
In order to intellectually distance my possibles from a purse, it does not have a strap that goes over the shoulder (it originally did but I took it off). It has a carabiner that hooks to my belt, pants or pocket. It works just as well, maybe a little better because a purse strap puts uneven weight on one shoulder and actually hurts after a while. Hooking to a belt doesn't seem to have the same negative effect.
What I like about this bag
It's relatively small and light.
It substitutes for a wallet.
It has a number of different sized pockets that are suited for medium to small sized items.
It has two sturdy D rings on the sides that make hanging things off it and hanging it practical.
It's exterior is very sturdy and has held up under everyday use.
If you can find a bag that does those things you'll do well.
What I don't like about this bag
It can be complicated to open and get to things in the deep insides.
The belt loop is vertical instead of horizontal. I would use the belt loop if it were horizontal.
Some of the lining is all that holds things in the top pocket and has fallen apart. (Fortunately my wife is a skilled seamstress.)