Close walking distance to Paulsplatz, several of the local cathedrals and other dining and shopping options. It is also a fairly short walk (10 minutes or so) to the Frankfurt Hbf (main train station) which is very convenient if traveling light and it can save you 10-15 Euro versus a taxi. The proximity of the train station also makes it a good jumping off location for trips to Munich, etc.
As with most European hotels the rooms can be very "efficient" with regard to size and amenities when compared to U.S. hotels. The Zentrum is an older hotel but it was clean and well appointed for the money. Since it is located across the street from a rather good sized shopping plaza and bierhaus, it can get a bit noisy of Friday and Saturday nights. Also, two additional things worth mentioning. First, don't forget to bring you plug adaptors/convertors for recharging your electronics! While some hotels will provide a "loaner" don't count on it. We had our own so I am unable to comment on whether the Zentrum provides them or not. Second, most German cities have a local tourist tax. Throughout our travels, it averaged about 8 euro/day and it is collected at check out.