I normally don't do many product reviews, but I have one of these bad boys I think they're pretty awesome. I typically use it on my motorcycle. The pocket on the back comes with what they call a "Universal Holster". Basically a couple straps of velcro to hold everything in place. The pocket will fit a full-sized M1911A1 and a spare mag.
It's not the biggest bag in the world, but it's one that I keep using.