TL;DR: Great experience, would use again.
The negative:
The place showed it's age somewhat, the decor was rather tacky, the sofa and bed could've been a bit softer, the fridge's door shelf was broken, and the bath sink/mirror was poorly lit. Lighting was centrally controlled through a dashboard between the bed and sofa, but it's hard to use from bed, and more so when dark. I didn't get the karaoke to work, but then again I was just curious and didn't spend much time on it. The TV had lots of free VOD movies, but seemingly with poor video quality and without English subtitles (even in parts where the cast spoke non-English). Also, it was slightly annoying having to hand off and retrieve the key at the reception every time I left and returned. Altogether however these things were minor.
The positive:
Big room and bathroom by Tokyo standards. Big hot tub with strong jets and a TV. Dry sauna right next to the bathroom. Bidet with even more features than what is usually found in Japanese bidets. A mini bar stocked with various alcoholic drinks and even a couple of free beverages. A small fridge for your own things. Free karaoke (see above). Free VOD movies (see above). Lots of free stuff to use/borrow/take, e.g. bath robes (borrow). Even more stuff freely available on request. Great service and very nice staff. Very central location, and although the entrance appeared a bit hidden in an alleyway, I had no safety concerns even at nighttime.