It is a great spot for a quick, authentic dip in real onsen water that is piped up from deep below Tokyo. The water is a brownish color and is medium hot, though not scalding. As such, for many this could be a decent introduction to the world of Japanese onsen, especially for those only in Tokyo, or only in Japan for a short time. There's also attractive murals of Mount Fuji on the wall.
The outside bath is the most tepid, the inside bath is hotter, and be careful where you step -- one, the surfaces are slippery and (of course!) the water is hot. But there's also a "Denki Bath" (or Electricity Bath) and while the Japanese occasionally go for this, it's entirely at your own risk.
Though the nearby Asakusa Kannon Onsen is more atmospheric, with creaky wood floors and scalding water, Jakotsuyu gets points for trying: there's a rotenburo portion that looks upwards to the sky (it's walled in) and a carp pond with real carp (don't dip in this, please!). At peak times (such as early evening, this bath can be packed.
Onsen buffs or those needing a quick bath can definitely count on Jakotsuyu. It's authentic, has reasonable prices and offers amenities for purchase if you forgot your own. The vending machine for tickets has English and while dour, the matron at the door will point you in the right direction. You can almost hear her thinking, "Please, please, Mr. Foreigner, be sure you wash first..." So make sure you do!