I customized it with zippers so it can be attached to my Berghaus Vulcan. It serves there as a sidepouch (the other one is a medic pack) and can be easily detached and you´re good to go.
It´s basically my Day Pack. In it are additional ammunition, comm, MRE and my poncho liner - plus space for... well stuff...
It´s different to using a regular small rucksack as a daypack and you have to get used to wearing it. But definately get one with a waist strap (or some kind of triangle strap system - see link). Don´t buy cheap or you will buy twice. The waist strap holds the pack on your back bust must be easy to open that you still can "sling" the pack on your front to access it.
I use the Hazard 4 Evac and as said changed some parts. Works fine now for almost 2 years.