Overall a great experience! David and the other reception staff are very kind and helpful, and speak fluent English. When we arrived, there was construction going on and the room we booked was being renovated, so they upgraded us to a larger room free of cost.
One thing to be aware of if you're arriving in Shinjuku via the JR line: this apartment/hostel complex is about a 15 minute walk from the nearest JR stop, so make sure you take that into account, especially if you have heavy luggage.
Would definitely recommend!