We stayed at the Duke of Buckingham for 2 nights, in the family cottage.
The room was well furnished, very clean and was very charming, in an older building behind the hotel.
The only "negative" aspect was that we had a bit of noise because the upper floor was occupied as well. (old buildings' floors tend to creak a bit) but it didn't matter too much as we were provided with ear plugs.
The staff was really friendly and helpful, so that we even went to eat at the restaurant twice. The food was nice and the quantities were very large. We would have liked to have a breakfast included in the price but we found other nice options in the neighborhood and were provided with charming adresses.
The hotel is really well located, in Old Portsmouth, 2 minutes away by foot from the sea front, on one of the main streets and 10 minutes from Gunwharf Quays.