I've never seen one I actually liked. The Ortlieb adaptor is well-built, but it's a heavy beast of a thing, and if you allow space inside the pannier for it while in bike mode, detracts from the overall capacity by a fair bit.
There seem to be better options in the messenger bag style, if that's an acceptable alternative.
Hbunnet's suggestion of carrying a rucksack inside a pannier has some merit if you can reasonably leave the pannier on or near the bike (perhaps along with bike tools/clothes/helmet/etc that you don't need to carry with you). Something non-bag-like like Bike Bins or a basket could be securely attached to the bike and unattractive to thieves.
I second the health warning about cheap gravity-powered fixings on Lidl panniers. I have one in an Ortlieb style that seemed good value at 12 quid, but after bouncing partway off on the maiden voyage, it's confined to storage of infrequently used bike stuff duty.