I found the 9 Hotel a very enjoyable hotel throughout.
The first impression is great, as we are welcome by a well designed hall and reception room matching what advertised in the hotel website and a diligent service even if not particularly remarkable.
The rooms I've tried (a single and a double at two different occasions) were very elegant if minimal, stripped off to the basics in a comfortable manner. The beds were delightful and the bathroom of high standing for this category of hotel. Cleanliness was remarkable as well.
Also, the 9 Hotel is very well located with most areas of interest in Paris within easy reach. The same goes for the main train stations and airports.
On the minus side, the rooms feel a bit small, lack some furniture and tend be noisy as they are right next to the corridor. As such I would probably not stay there more than 1-2 nights unless I would be staying at one of the premium rooms.
Finally, at its price, the 9 Hotel is a very good deal and I'd therefore would not hesitate in recommending it for a short stay in Paris.