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.