We stayed as a group of six adults plus a baby in July. The hosts warned us when we booked that it would be a little tight (which I appreciated) and they were right. Two of the bedrooms are tiny, but the master bedroom is large and the living area is comfortable and has sufficient seating. Overall, we all fit and made due.
The location of the apartment is fabulous. Hampstead is a great neighborhood and the Tube is a very easy walk. We loved the location.
We all felt like the apartment itself could have used a little extra attention. It's advertised as "shabby chic" but we unfortunately found it to be a little more of the former than the latter. For example, it was uncommonly hot the week we were there, but the standing fan was missing its legs and couldn't stand up properly. Fortunately, there was a good breeze at night.
That all said, the apartment worked out well for us. The location probably outweighs any nits about the apartment itself, and it's not easy to find a place that fits that many people. The hosts were also very friendly and responsive. So it was a positive experience overall, tempered only somewhat by the issues mentioned above.