I use a photographer's vest and cargo pants for my stuff. As others have pointed out, the vest isn't as hard on my back as the backpack I used to wear, and backpacks are notoriously hard to get stuff out of quickly.

See
http://lifehacker.com/software/screenshot-tour/show-us-your-go-bag-part-2-292427.php
for photos of a variety of go-bags, go-pants, and go-vests (this is part 2; it has a link to part 1).

Ammo does fit in photo vests, by the way:
http://lifehacker.com/photogallery/show-us-your-go-bag-part-1/2388489

A photo vest is a real article of clothing, so I get more use out of it than the thing that's called a "vest" in the phrase MOLLE vest. I wear it in airports with my walkie-talkie, water, TSA-passable survival stuff, paperback books, magazines, etc., without attracting unwanted attention by looking like a paramilitary wannabe.

The problems with a photo vest are - it's not padded, so stuff pokes you and you have to be careful setting it down or sitting down if you have breakable stuff in it; if it's hot, it makes you sweat less than a backpack, but you still sweat; it can be hard to get into if you have a bulky wrist watch that catches on the armholes; although the I much prefer the weight distribution in the vest over that of a backpack, it's still heavy; it can be a pain to sit in when you have to sit without taking it off - depends on how you pack it.

One nice thing is that it's the equivalent of another carryon bag when you fly. I have a rollabord and a small toiletries bag that gets me waved onboard without comment, but they don't pay attention to what's in the vest, and it's another bag's worth of stuff that I don't want to check.

Everybody's mileage varies, so what you need won't match my needs. But my photo vest does yeoman's service for me as a go bag, travel bag, and - gasp - photo bag. :-> I have two vests, and I recommend trying them on and loading them up before buying if you go the photo vest route. Some vests have collars that are too far forward or too high or something, and they press on my neck really hard, making them unuseable to carry a load.