Our stay here was one of the best values we had in Germany.
The good: Location is excellent and walkable to many sights and public transit. Staff friendly and helpful - we were able to check in over an hour early, and they hold baggage. Bathroom was a decent size and room contained helpful amenities that not all German hotels do (kettle with tea/instant coffee, shower gel/shampoo, minifridge). AC wasn't high-powered, but made the room cool enough to sleep with closed windows in early June. Two types of pillows (smaller and larger continental style).
Limitations: The rooms were likely designed for business travelers in the 80s/90s, and the decor shows this. Didn't matter to us, but might to some. We had what seemed to be a larger room (listed as for business travelers) on the front of the building. There was some streetnoise despite good windows (first floor), but the noise machine we brought helped out. Inner rooms seem smaller but might be quieter. Bed was not the most comfortable (older spring mattress) but worked.
Overall: If you need a place to spend a night or two in Frankfurt, and are ok with an older hotel, this option cannot be beat for the price.