It took 10 minutes to reach this hotel on foot from the Mainz Hbf station. Location is in a residential neighborhood in the "Neustadt" quarter - not upper-end by any means, but not nasty either. There's a Rewe grocery store one block away plus a few scattered small-scale places where you might get prepared food or coffee. The reception has limited hours. Staff there was friendly and helpful.
I stayed one night - had a "standard double" for use as a single, including breakfast, with private bath, desk+chair, dresser and wardrobe, a balcony with chair, no a/c, no coffee-maker, for € 74.70. The room and bath were very clean and tidy - the room was huge but very basic/utilitarian. The view from the balcony didn't add that much to my stay. I slept fine at this place as the noise level with windows and balcony door shut was near-zero. Breakfast was buffet-style, with a small but comfortable breakfast area, and while not extensive still had decent variety. Weak spot in the breakfast: the coffee machine was kinda low-end, not the fancy bean-grinding sort, probably instant-based. Not at the top of my list for Mainz, but I'd certainly look at staying here again on some future trip. In summer, I might opt for one of the large multi-room chain places that offer a/c and compete somewhat close to this price range, but otherwise, I'd gladly stay again at the Moguntia.