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

So there are few things people complain about here - like small elevator. Well, it is small, but we had smaller on this same trip. You can fit 4 (friendly) people in this or two with a bag each. Worked well for us. Now the shower is a bit small for someone large. Need to be careful how you move around in there.

The desk and breakfast staff were very friendly and helpful. We came early and they stored our bags as usual. When we came back, they'd taken them all up to the room for us.

Decor is very modern. Wireless internet is fantastic - best I've seen in a hotel anywhere. I had it up with my VPN up for 4 days straight with no drops.

Breakfast was very good - buffet with a few hot items and a lot of other things including really fresh fruit and breads.

Location? Well, it's great for getting to the station. It's a bit close to the sex club area so don't take a wrong turn at night. Room was very quiet and beds comfortable.

All in all, very happy with the place.