My first review got turned into the description text instead, so here's an update.
Usually five soups on offer in addition to salads and bagel sandwiches. At least one soup is vegetarian, sometimes vegan. All veg items are clearly marked. The restaurant is small and pleasant; it used to have unusual seating, but has normalized now. Its location is right next to a large Alnatura, a German natural foods chain (sort of like a Whole Foods in the US). If you're in transit through Mainz, reckon on about 15 minutes to walk there from the main train station. Very close to the pedestrian and tourist areas.
Cons: limited menu, accompaniments to soup are hit-or-miss