I stayed here for couple nights. It is located at the end of red light district (which does not bother you at all), very safe, next to train station, close to waterfront and within 15 minutes walk of very bohemian area of the city. The old city center is like 20-25 min by walk or just few minutes by car.
Rooms are nicely appointed, quality fixtures, great windows, very modern, comfortable bed, nice bathrooms. There are lots of yellow colour in interior, so if you hate yellow, stay away:) For me it felt good in grey rainy Hamburg.
Service was impeccable.
I would stay in this hotel again.

The service in this hotel is a breath of fresh air. They care. It was my boyfriends birthday and they brought a bottle of prosecco, a home made cake and a card with handwritten wishes of all staff. It started the day extra special. The exterior and interior of this hotel looks gorgeous and unique. Definitely recommend it if you want a nice relaxing stay.

We stayed in the Onyx for 3 nights. The hotel is new, modern, comfortable and very clean. The staff is friendly and helpful and there is very convenient and cheap parking available under the hotel. We had breakfast in the hotel only one morning. The 'continental breakfast' is fine, but not very special. For the same price you can have breakfast everywhere in Hamburg. The hotel also has fitness and wellness facilities. We went there on Sunday afternoon and it was lovely!
We had a standard room, and it was very nice and big enough for a short city break. We didn't like the area very much as we're not into bar hopping and nightclubs. Also it was difficult to find a nice restaurant in walking distance of the hotel. The harbour is close and there is a U-bahn very close, we walked most days to the centre and it is doable but you do have to walk quiet a bit.
All in all a great hotel and if you're in Hamburg to get a taste of the nightlife an ideal location. If you don't mind the distance to the centre this hotel has a lot to offer.

Stayed 4 nights, love the location 1 minute to the U3 station St, Pauli, right at the beginning of Reeperbahn, good distance from the noise, but close to the action. the staff were very friendly, and helpful, I asked for a late check out, and they happily allowed me to stay till 3pm, this saved my day. highly recommended hotel. it is about 25 minutes walking distance from CCH, or 3 bus stops (bus line 112) the bus run every 15 minutes.
The rooms are large compared to the European standard.
Only one minor issue, the quilt is relativity small, they should get bigger ones.