We've stayed at Mercure's before and we were pretty disappointed with the quality of this particular hotel (especially for a German location in a large city). The rooms desperately needed an upgrade/renovation, as they felt like a cheap US hotel chain (think 2-star U.S. motel). We booked a King room, and got a Queen bed that felt like concrete. The bed had 2 ridiculously small and flat pillows on it, and the sheets were cheap and scratchy (they actually triggered an allergic skin reaction for both my husband and I). There were stains on the carpet, and the bathroom smelled like urine. The room was also incredibly hot, and you had no control over the heat in the room. It also didn't help that we got caught up at check-in with a big tour bus filled with pushy and impolite seniors. The whole thing made for an all round disappointing and negative experience.
I must say though, the staff were pleasant and professional, and I must give them credit for that! The breakfast was also pretty good. Overall, this felt more like a 2 star hotel, not a 4 star "Mercure-quality" hotel though, and wouldn't recommend it for couples or singles that are looking for business-class or 4-star accommodations.