This is a decently sized apartment with the general essentials. Although I didn't use them, there were toothbrushes, toothpaste, and various outlets/chargers (including a travel adapter) available for use, which was a nice thought. More than one towel, too. The tatami area is nice and you have the option of using a Western style bed or a futon on the tatami.
If I had to say, the kitchen was a bit old and you can see in the other rooms listed (Smart Stay Tokyo) that the provided rooms in the same building look much nicer, so you may want to check on that end as well. Also, the bathroom consists of the shower and bathtub in the same area, so you would have to take a shower first, then leave and wait for the bathtub to fill up before soaking in it. Also, the water pressure from the shower is rather pathetic. I'd have to say that was my main disappointment with the room. Additionally, it would have been nice to have a dryer as well.
There is a nice laptop shelf (though a bit narrow) along the wall and plenty of outlets, which was useful.
Location-wise, this is located in a rather quiet location of Shinjuku (apart from the occasional ambulance noises at night). It is right next to a bus stop and a Yayoiken, and close to a market. If you have some business in Ikebukuro or near Shibuya, this is rather ideal. Additionally, it is a short walk away from the Shinjuku 3-Chome Station, and you'll save a fair bit on commuting fare compared to if you were located a bit more remotely. There was a bit of an inconvenience heading to the room from the JR Shinjuku station though, granted that's more the fault of how large it is and the amount of people using it. Other than that, this is located in a bustling area with many familiar shops nearby, so I may consider staying here again in the future.
Check-in and check-out was smooth, though the lockbox on the doorknob is a bit thick and makes the handle hard to turn at times.