useful but not perfect
This has a velcro attachment in the back that fits nicely on the ergo carrier's waist strap. Since your shoulders are in use already, it's great to have a purse-replacement right where you can reach it for things you need while the baby is still on your back, like your wallet, etc. It's large enough to be useful while small enough to not load you down. Just using this pouch and the pocket in the back of the carrier, without using the back pack attachment to the carrier, you can fit everything you need for a few hours out with the baby - daiper, travel pack of wipes, jar of food, baby spoon, flat packing bib, nursing cover, wallet, keys, and larger size cell phone. For a longer all day outing, I got the backpack attachment as well.
The only problem with it was the quality of the smaller zipper for the extra front pocket. The main zipper was larger and worked fine, but the small zipper in front immediately fell off it's track the first moment I unzipped it and some of the tiny plastic zipper teeth bent, despite unzipping it gently, so it never once worked. However, I felt like the small front pocket was unnecessary anyway, so I am still happy with the purchase.
Prossize, fit to carrier
Conszipper quality of smaller zipper