I really enjoyed my stay here, with one caveat that I'll get to in a second. But first, the pros: the location is absolutely incredible, the apartment's a good size (I stayed in another airbnb in Tokyo that felt like a very small cage), and it's super clean. Really, I can't stress the location enough -- it's a 2-minute walk to the closest local subway line and a 6-minute walk to the JR Yamanote Line, so if you have a Rail Pass, this apartment's a slam dunk for getting all over the city. And despite being in the middle of the city, there's not a ton of noise; so if you're looking for something in Shinjuku that isn't IN SHINJUKU, this place is great.
The one issue that I had was two small things, actually, that compounded into one -- there aren't any blinds on the windows, and the tatami are rather thin. So as another reviewer mentioned, sleeping here isn't the easiest thing in the world, especially after a few days. But if you're looking for something quick and easy in a nice part of Shinjuku, or especially if they thicken up the mats, this place will absolutely do the trick.