My wife, daughter and I stayed at Hotel am Berg for two nights in November. The staff were incredibly friendly (the day-time clerk, whose name I can't recall, was a wonderful help when it came to navigating the subway and knowing which stops would get us to and from the hotel to the various places we wanted to visit; he even helped us find a shop that would have a relatively obscure item we were interested in). The room was huge. The decor was a little dated, but that honestly added to the charm of the place.
It's located in a pretty quiet residential neighbourhood, but very close to a subway station, so getting around the city was easy despite the hotel not being near the major retail and tourist areas. If you don't mind taking the underground, this location is probably better than being at a hotel "closer to the action" just because it is so much quieter and easier to get a good night's sleep.
If I am ever in Frankfurt again, I would definitely stay here again.