My boyfriend and I went to Hamburg as a stop on our way to Amsterdam, but we were just blown away by Hamburg, greatly in part because of Clipper. Before arrival I had emailed with the staff about parking, and they sent me a QR code to scan at the garage upon arrival (this was awesome, as we didn't have to check in before parking).
We had reserved an apartment with a little kitchen, and this worked great. We loved living in Hamburg for a few days and we felt so at home.
You can check in with the number of key cards that you want.
The placement i St. Pauli is safe and close to a big, neat Rewe (grocery store) and the habour which is great for walks and a habour tour.
We unfortunately melted the plate cover for the microwave (by mistake) which the receptionist just laughed off with us and didn't even want us to pay for.
We will be back at Clipper, that's for sure.