My girlfriend and I thoroughly enjoyed our stay at the mansion! The place has everything a traveler could need, and William's hospitality is second to none.
We didn't arrive until almost 10 at night, but check-in was no problem with the key in the lockbox on the front door. The mansion itself has seen better days (the staircase leans various directions), but the apartment itself is well maintained. Once inside the place was exactly as described and even included a complimentary (and in my case much needed) 1-gallon bottle of water in the fridge!
The bed was comfortable and supportive (which turned out to be somewhat important after I threw my back out my first morning there), making it easy to relax after long days of walking around NYC. The neighborhood can be a little noisy at times, but not to a level we ever found annoying or uncomfortable (it's NYC - you're gonna hear sirens no matter where you are!).
The bathroom is spacious and bright, and everything worked fine. The apartment doesn't have a true kitchen, but there is a microwave & plates/cutlery to heat up leftovers as well as a tiny sink and cleaning supplies to tidy up afterwards.
The neighborhood was fairly convenient to most things we wanted to do. The Brooklyn Botanic Garden (which is amazing, btw) and the Brooklyn Museum are within walking distance of the apartment, and everything else was somewhere between 15-50 minutes away via subway. There are plenty of restaurant options nearby (several of them on Nostrand) as well, especially if you want some Jamaican food.
Finally, William is a fantastic host. He checked in on us several times and gave us several good recommendations (Lula Bagel was especially yummy!) while we were there. We never needed anything for the apartment, but if we had he would've been ready for it very quickly.
I definitely recommend staying at this apartment in the mansion. The accomodations along with William's hospitality were first-rate - thanks again!