Amazing location and the onsen was spectacular! I would highly recommend to anyone staying in Tokyo. Shinjuku itself is a very fun neighbourhood, with lots of places to eat, fun little bars and lots of streets to explore. Onsen Ryokan Yuen is close to three train stations which makes exploring the rest of Tokyo and your ongoing travel an absolute breeze.
The staff were very helpful and friendly, the room was small (typical for Japan), but it was immaculately clean and had everything you needed for a stay.
The onsen was absolutely spectacular! Rooftop views of the city, perfect temperature and a great way to unwind after a day of exploring. I would definitely stay here again (if not just for the onsen).