I stayed here with my family (4 of us in total) in January 2020 and this place was overall great! It was in a very quiet and peaceful residential area of Shinjuku, about a 7 to 8 minute walk to Waseda Station on the Tokyo Metro Tozai Line, and about a 10-12 minute walk to Nishi-waseda Station on the Toko Metro Fukutoshin Line.
It's pretty much a 30-45 commute via subway trains to any major Tokyo destination, which I think is not too bad!
The space itself is comfortable. Totally great for people who are gonna be out and about all day but wanna come back to a nice and cozy place to wind down and sleep.
The only problem we had was with the WiFi. It always worked fine in the morning, we'd wake up and use it without any noticeable problems. But whenever we got back in the evenings (we usually got back around 8 or 9pm), it would be significantly slower. For those of you who care to know, I did a speed test during the morning and it was downloading at about 20 megabits per second (pretty standard for the most inexpensive household internet plans out there... streaming Netflix standard definition videos require at least 3 mbps, and 1080p HD videos require at least 5 mbps). At night, I did another speed test, and it was downloading at about 0.6 megabits per second, which is 33x slower. I don't know if that's intentional (i.e. the owner of the Airbnb put some restrictions on wi-fi usage at night), or if there's some sort of daily quota and a neighbor is stealing the wi-fi all day. Whatever the case, I've let the owner know and hopefully he fixes the problem.