I've been there for lunch, but it should be great for dinner too:
Carls Brasserie, http://www.carls-brasserie.de/
Located in the „hafencity“, it offers a great view of the harbour, beside its tasty modern french-style food.
Restaurant Central
Lange Reihe 50
20099 Hamburg, Germany
+49 40 28053704
Great, relaxed restaurant that I have been back to many times and love it for the fact it doesn't have any of the failings so many other decent looking restaurants have.
Bistro Vienna
Fettstrasse 2
20357 Hamburg
+49 40 4399182
http://www.vienna-hamburg.de/
This was the best food I had in Hamburg an came in a probably half the price of most the restaurants I ate in. It's cramped, busy and you'll leave smelling like you were sat in the kitchen but the food and atmosphere are great.
I have been in Panthera Rodizio http://www.panthera-rodizio.de/
It was a very good restaurant to eat good meal.
I suggest you to reserve before you go on Friday and Saturday evenings, otherwise it's very hard to find a free table even for 2 people.
Gröninger Privatbrauerei
Willy-Brandt-Straße 47
20457 Hamburg
http://www.groeninger-hamburg.de/index.htm
German food and good homemade beer!
Café Paris, Rathausstr. 4
A classy brasserie with tasty food.
http://www.cafeparis.net/