A solid, no-frills option for those who wish to stay in Shinjuku. Convenient to a couple of different metro stations, easy check-in with clear instructions, and quiet. Be aware that check-out is at 10 am with no option to extend.
A couple of other notes:
You will need to climb a narrow and slightly twisty flight of stairs up to the flat.
There's a very bright exit sign in the flat: It's probably a requirement but it's bright enough to disturb all but the easiest sleepers. Pack a sleeping mask.
If you're accustomed to using a washlet, this flat does not have one.