We did spend two nights at the Mercure in Hamm. It has a 24h check-in however they did not know that we are arriving late even if I told them by phone upfront and they made a note for the responsible person. But not a problem that the reservation as the hotel wasn't fully booked so we got a room. Reservation was for a twin bed and we got a double bed but we didn't care. We asked for a wake up call and I saw the staff writing it down. But there was no call - that's a no go. Also the card reader wasn't working at the check out. Took us quite a bit until they offered to make a copy of the credit card and charge the rate at later time.
Beside those points the staff was nice and friendly. We were asking for a late check-out and got one extra hour.
Rooms were clean but felt a bit aged. The bath tub was a bit dirty, rest of the bath was good.
The breakfast was a nice selection of bread, cheese, meat, eggs, sausages and fruits.
There was some renovation going on. I think that's a good idea.
In general we can recommend that place even if it was not 100%.