I like hiking and I like biking, so if I'm on a bike tour I'd like to have the option to get to a scenic area and go hiking for a bit. I'm gradually accumulating more gear and I'm starting to look at panniers, one pair at a time. I want some solid touring panniers where one of them can be converted into a backpack that's comfortable enough for hiking all day or even for several days with a small load. I know that by doing that, you kind of have to compromise on comfort, but that's how it goes. I guess my other option would be to get some ultralight daypack and just take that along on tours.
Anyway, the two options I've found by googling are any Ortlieb panniers, which can apparently be turned into a daypack by adding a harness you buy separately, and the Arkel Bug, which seems more intended for commuting. From these two, I'd really favor the Ortliebs, both because they're waterproof and because for some of them, the price per pair is barely more than the price of a single Bug.
I haven't been able to find any reviews of the Ortliebs used as backpacks, so I'd like to hear about any other options people know of and especially of people's experiences using the backpacks, since I'd like to know which ones are at least decently comfortable.