Stayed at the Abbey for three nights with my teenage daughter; we were using the room as a base for shopping in the West End and a show at Hyde Park. The location is fabulous, with Nothing Hill gate tube station a short walk away and Hyde Park a few minutes away.
The staff on arrival were extremely friendly, helpful and attentive, and this continued throughout our stay. The room was small but comfortable. The decor is dated, and the rooms would benefit from a lick of paint. The nature of the building means that walls can feel quite thin and our neighbour insisted on having the TV on full volume most nights until quite late but this isn't the fault if the hotel.
Overall, as an affordable base to explore the heart of London for a few days, this is a very good choice.