The Hotel Concorde is a very small hotel that is only a half a block from the Karlstrasse exit in the Frankfurt Hauptbahnhof's lower level. The room size can very greatly. The smaller rooms are very small. The larger rooms are quite comfortable though (see pictures). The staff was friendly and helpful, and when I got there slightly early they served me a Latte and some pastries while I waited for my room to be prepared. The room features mood lighting and a glass shower. It would be a great place for a romantic getaway. On the downside, being so close to the main train station, you will encounter the occasional panhandler on the streets nearby. It is an easy walk to the Altstadt, or you can take one of the many streetcars heading in that direction. As with all German hotels in major cities, room prices will vary greatly depending on whether there is a trade show in town.

I stayed at the Concorde for 3 nights during a business trip, since the other hotels I normally stay at were booked due to an exhibition at the Messe. I based my stay on the ratings on TripAdvisor.
My room was teeny teeny tiny. However, the hotel is recently remodeled and relatively modern. The bed is simply awful. I don't even think it's a twin size, and a very cheap mattress. It felt like I was in College Dorm. Breakfast was included and ok.
I've stayed in the Main Train Station (Hauptbanhof) area before, and I do not like this area. It is full of drug addicts and other recalcitrants who frequent major train station areas. The place I do business with is not far away, but this is the last time I will stay in this area.
Despite everything, what they say about the staff in these ratings is absolutely 100% true - they are exceptional. However, I also had questions about hotel security. For being so close to the main train station, there were times I would walk in the hotel in the evening after dinner and no one would be at the front desk.
If you're on a budget and want a place to stay, I guess it is ok. But I won't be going back.

My single room was tiny. But the staff was helpful and the location was just what I needed, very near the Frankfurt railway station

We arrived early morning and was given our rooms, they are small but have all you need, excellent service from all the staff and breakfast is fantastic! Easy access to trains and underground, supermarket straight across the road. Anne, Melbourne Australia