The bag sticks into the medium zero messenger bag with ease. It is very useful in storing small items (flash drives, phone charger, SD cards, etc.) that I would be afraid to store in the bag's external pockets. I wish it had an internal pocket or divider to help me sort out the little items. Other than that, it is very solid and the quality is awesome.
The pocket is great for the smaller (checkbook, earphones...) stuff that tends to get lost in the larger bag. The material is tougher and heavier than I expected from the picture. I didn't realize that the Zero bags had heavy velcro strips mounted along both the front & back (upper & lower in back!)- so, you can move the pocket around to a place that works best for you. I wish that the Simple Pocket came in different colors and some different sizes as well, but overall great addition to the bags.
Easy to remove and put back in. Perfect for pens and other things that could leak into my actual Zero Messenger.