Kotaro's place was perfect for us. 4 not so small people could easily fit in there, split in different rooms, and though the common space is quite small, this is Japan ! No place is that huge, especially in such a great area like Shimo-Kitazawa. And when in Japan, you don't really stay inside, you go explore - or at least should you - so that was perfect. The place was clean, quiet, everything working fine, wifi is provided and can be taken out of the apartment... It is tricky to fine the first time as it is located in one small streets amidst other similar looking small streets, but on the 2nd day we didn't even need google maps to walk back and forth between the apartment and the subway station.
In fact, the subway is close enough to walk and it makes you go through a pedestrian street that has tons of restaurants, bars and shops. It's opened til late and provides a perfect late night or after-last-subway entertainment close to your home. This is the musician district, so expect lots of rock-looking cool Japanese folks carrying guitars and bass and other enjoyable instruments. Friendly neighborhood as you can expect.
Kotaro himself is a musician and an overall great guy, speaks good english which makes it easier for first time travelers to Japan, and he is ready to help if needed. I had some delivery from Amazon.jp and he helped me out to call the delivery guy and made sure I did receive my order. Feel free to contact him, he replies fast and is always helpful !
I actually think I shouldn't write such a good review for this place as you guys will most likely want to rent it out, but I am planning to make it my official Japan place to stay since we all felt in love with Shimo-Kitazawa, and Kotaro's place is definitely a great place to stay over and discover the area.
Go for it !