Glad for the above feedback on the Ortlieb offering like this. I've been curious about it too. Instead I picked up the Sunlight Fortress Backpack Pannier a while back. Which is also a huge compromise. On the rare occasions when I use it I run leather strap through its top handle to secure it to the rack as it has a tendency to jump ship otherwise. The pad and straps take up a fair amount of space inside it when it is on the bike, and it is only a medium sized pannier to begin with. As a backpack its not the most comfortable thing. I haven't taken it on any hikes, but I have worn it for several hours a number of times. I'd give it an "OK" rating, providing you strap it to your rack and don't expect it to be too comfortable/go too far in it. It looks like they discontinued it, but they pop up here and there including ebay.

You'd think somebody would be able to execute something along these lines a little better. I know it is always going to be a compromise, but there's got to be a better way than everything I've seen so far. The Sunlite could be greatly improved just by having different shoulder straps that were more ergonomic. Of course part of that might be that I'm 6'2" and barrel chested, the first time I put it on I got the impression that it would fit someone much smaller than I much better, but I still think the straps would be less than ideal. Their shape is weird.