This tri-cycle is so evidently shouting "Tourists Only". Standing in the Haupt Market Square, a couple of tricycles wait for tourists everyday to show them around the old town area. It is easier for them to cycle around the 2-3 km radius of old town area.
The only tough part is uphill to the Nuremberg / Nurnberg Castle. These tricycles are small and cute in a very European quaint way. They have a top cover against the sun and a windshield for the driver or the peddler. And these operate only in Summer. Ofcourse, I can't imagine tricycling in snow :(
My Two Cents
I would say take a tour for the first time visitors. If this is your first time in a small European town, it could be fun.
It could be a bit heavy on the budget traveler, fret not! That's why you are in a "Small European Town". The old town area is definitely walkable and a pleasurable walk too.