I attached this to my PRVKE EDC, I can stuff a set of gym clothes, an extra notebook or larger Sony headphones, depending on the day. It adds some flexibility to a smaller form factor pack, without feeling oversized. The hard backing of the pocket also makes stuffing a rain jacket between the Pack and the Pocket really easy. The pockets and bands are great for securing smaller items, and don't snag when an object is awkwardly shaped.
I thought I would end up taking it off and on, however the Pocket become part of the pack now.