Our stay at J place was great, the place is very central, it's easy to walk anywhere from there.
The apartment is above J's wife store, but we got two sets of keys and had no problem getting in or out anytime (The store itself has some lovely items, be sure to check it out).
We lost our flight and had trouble letting J know (we had no internet), when we finally arrived, he was understanding and helped us get to the apartment nonetheless.
The only problem we had was that the heater on the bedroom was not working and it was kinda cold in the room because of that.
Would recommend to anyone looking for a walkable place to stay in Amsterdam!