When i was considering buying the backpack accessory, i coudln't really find any information on it, so i thought i'd post my quick assessment of it. Sorry about the picture quality, i'm really bad at photography.
Here's a picture of it beside a Back Roller Classic:
The red x's are where the backpack accessory connects to the pannier.
At the top it uses the standard pannier hook thingies, and at the bottom, there's a slot on each end of the pannier, and an insert on the backpack accessory's straps, which slide in pretty tightly.
Here's a picture of it attached to the backpack.
And here's me making a stupid face with it on my back.
When i first took got it, and put it onto my pannier, i thought "huh, this is pretty cheap, i'm not very impressed", and everything seemed sort of "loose". But then i remembered i'm in no way qualified to judge a backpack accessory by looking at it, so i tried it out. It's as comfortable as any backpack i've used before. I used it on my first cycling trip and hiked around for 2 days with it relatively heavy, (it was carrying food and water), and it was comfortable the whole time.
I also don't have my top pannier clips in the default location, they're actually way more to one side, but that didn't seem to matter.
One slight annoyance is that it takes up a decent amount of room in the pannier when you're not using it, and it's a bit of an awkward shape because the of the straps and stuff.
One other annoyance which took me a day to remedy/get used to, is that if you lift it with the black strap on the pannier, it opens the top clips and which detaches it from the pannier. You have to lift it with the grey strap that's on the backpack accessory itself.
Does what it's supposed to do.
Comfortable with the pannier clips off center.
Lifting with the wrong cable detaches it from the pannier at the top.
Takes up room in the pannier
I would recommend it to someone wanting the features it provides. I think i paid $30USD.
*disclaimer: I don't know much about backpacks, i've never done any sort of backpacking or anything.