Regensburg takes its name from the Roman fort Castra Regina, founded during the reign of Marcus Aurelius, so it is one of the oldest cities in Germany.
I've only been there once, and spent most of my time at nearby sites like Walhalla, so I can't really speak to the city's Gemütlichkeit, but it is a pretty popular tourist site.
It has the largest baseball stadium in Germany, home of the Regensburg Legionäre, though that doesn't really mean much since baseball is not popular in Germany. Its capacity is about the same as an Independent team in the US, 3000+.