This onsen was featured in the American Way Magazine for Aug 2006. That's how I found out about it and decided I wanted to try it out for myself.

I will try to be as thorough as possible.

Getting There:
This onsen can be reached by train from Shinjuku or Tokyo station (actually all of them fyi Use [--] for train routes). If you buy the Japan Rail Pass before you go, you will be able to use it all the way to the Onsen and back. Travel time from Tokyo Station to Minakami Station (the station closest to the Onsen) is approx 3 hrs. Once you arrive at Minakami Station you need to exit out and take a right to find the bus. The Takaragawa bus leaves at 14:25 and 16:25pm and will take about 30 mins to get there. This bus will cost you approx 1100 yen per person so BRING EXACT CHANGE. Tip: For those who are not so adventurous in their choice of Japanese food I advise you to bring many snacks and bottled water before you leave the Tokyo area. Once you arrive at the Onsen, bottled water is 170yen ea and there are very few non-japanese snacks available. They do have small cans of Dreyers ice cream for 360 yen ea for a late night snack.

Leaving There:
Returning from Takaragawa onsen to Minakami station: Upon check out, the front desk assistant will drive you to the nearby bus stop 1 mile away (free ride there). From there, the earliest bus is 6:50am, and then 7:42am and they are always on time. This 30 min bus ride will take you back to Minakami station where you can catch the JR Joetsu line back to Takasaki station and then on to Tokyo Station or wherever you are going. NOTE: If you plan on catching a flight from Narita airport the same day you leave, the trip from the Onsen to the airport will take at least 4 1/2 hours and that is catching every train as quick as possible. Plan ahead for enough time. The Narita Express train from Tokyo station to Narita airport takes about 1 hr.

Check in at onsen:
Be sure to bring your confirmation number with you and your name. When you walk in, remove your shoes and slip on the sandals. You will pick up your tennis shoes when you depart. You will sit down and have some green tea with a staff member who will give you your yukata (cotton robe), sash, and another coat to go on top of it ( I forgot the name of it). They will give you an overview of the property and try to answer any questions.

Guest Rooms:
We stayed in the Osenkaku building. It was very authentic and nice. Flat screen tv, minibar, small sitting area, windows that open, screen doors, full bathroom with Japanese soaking tub, nice toilet with seat warmer, toiletries, hairdryer, and very comfortable futons/duvets. There is also a secure safe and lock on your room door. The rooms are very small and you sleep on the floor (futons with duvet comforters and rice pillow)

Hot Springs:
ABSOLUTELY amazing! Be sure to use the shower rooms before you enter the hot springs to wash your entire body with soap. Rinse completely as you do not want to bring any suds into the hot spring water. Men are supplied small towels (enough to cover your parts) and women can use a normal size towel to wrap around them. This is all personal preference though. Some go completely nude and some don’t. It is fascinating to see the difference between American and Japanese acceptance of public nudity. It’s great! If you are worried about being nude, once you go you will see it is not an issue at all. For women, there is a women-only outdoor spring. But the majority of hot springs are mixed bathing.

Food:
Dinner is served in an elaborate presentation in your room around 7pm each night. Our main course varied from fish that you cook on a burner, sashimi, Japanese pickles, rice, bear soup (very good), mushrooms, rice, raw horse meat, noodles, and other authentic items. In the morning, breakfast begins at 7:30am and you can opt for a Western Style Breakfast (let them know at check in). While we were there, we had both Japanese and Western Style breakfasts. For our American tastes, the Western style was better than the Japanese one. It consisted of a small sausage, hard boiled egg, salad, vanilla yogurt with raspberry sauce, and corn chowder(very good). The Japanese breakfast included rice, sticky beans, salad, sweet egg omelet, nori with soy sauce, and baked fish. I have to admit, the change in food is truly a big difference so be prepared. Again, for those who’s tastes are not as adventurous as others bring snack chips, food, and bottled water.

Staff:
I just want to mention how incredibly nice the staff are at the Onsen. Most of them live in dormitories on the property and work very hard to keep this place beautiful. They were always eager to understand our English and attempts at some Japanese words. Although tipping is not customary in Japan, we tipped our hostess because of the wonderful job she did.

Take your camera! There is not much to do after you soak, sleep, and soak some more in the hot springs. The TV only has a few Japanese channels and there are no other activities to keep you occupied. If I returned to Japan I would come here over Tokyo without question.

This is one of Japan's finest outdoor onsen and truly an experience that you will remember for the rest of your life...I know I will!

We stayed at the onsen's on-site ryokan (Japanese inn) and it was fantastic. We had dinner served in the room, which was wonderful...and then walked outside to the outdoor baths (three unisex, one female-only). The baths were wonderful and words cannot describe how great the water/atmosphere felt!

The manager spoke good English, though I was happy that my Japanese is reasonable when dealing with the rest of the staff, who spoke little to no English. Getting to the onsen by public transport is a bit of an adventure in itself, as it is in a very rural location...you have to take trains and buses, so you have to be dedicated!! But if you make the effort to get there I don't think you'll regret it. :)

Very recommended!