Me and my boyfriend stayed 12 nights at Guillermo's place in Williamsburg. He was a great host and really communicative during our stay. We had some problems with the Wifi and with a broken blind and both were fixed really fast with accurate updates about the progress. He even payed for a 4G SIM card to bridge the no-Wifi gap! The apartment is located close to the Montrose Av L-train station and a 20min subway ride will take you straight into Manhattan.
The neighborhood is a bit rough but we had no problems with crime whatsoever. There was a party going a couple of houses down the road during our last weekend there which got a little loud. I've read here that other people complained about it too so I guess it's a bar or a concert venue of some sort. The apartment itself could use some AC tho as it got really hot there during our stay but that also could be due to one of the hottest NY summers in years with temperatures of over 35c.
All in all we had a great stay and we would definitely stay here again should we visit NYC another time.