So, have had a few days playing with the bahn bikes including the morning commute over the Rhine.
It's a great system, although it takes a bit of getting used to and there's not much margin for making mistakes (say if you type in the wrong bike number in the app you can't undo easily then have to call the German call centre and hope they can understand you)
But when it works, it works, the bikes are usually in the vicinity of where the app says they are but you sometimes have to scout about a bit as they aren't actually fixed to anything, just meant to be left with the wheel lock on at 'major intersections'
Cycling in Cologne is really lovely though, lots of bike paths on the pavements, traffic that understands its position in the road chain and it's not too hilly, although getting over Deutzerbruke takes a bit of legwork.
Seen more of the city this time then the two previous just by want of having some transit.
The bikes are full suspension for reason, very bouncy, kind of like riding a sofa balanced on a pogo stick.
THe one thing is, it's not hugely cheap €0.08 does actually add up, think I've racked up about €20 of usage :*) Still cheaper than renting a bike, and I don't have to worry about it getting nicked!