We went to the Sozankyo ryokan, in Uchinomaki (North-West of the caldera) in April 2012. I assume there are buses going to Uchinomaki, but I'm not sure; it's possible one has to alight at Uchinomaki train station, which is not in Uchinomaki centre either. The information can probably be gathered in the Aso Tourist Office, which is also very handy to have information about which trails are opened up on the volcano. The Aso Tourist Office is actually at the back of the big grocery store/food market (good local produce by the way) in the building next to the Aso train station, in front of the car park. If you are driving, it is not that easy to find the ryokan without a Sat-Nav but if you get it right, it should not take more than 10 minutes: from Aso train station, go onto road 212 to Uchinomaki. At the first traffic light of the town (when 212 road goes right), take left on to road 149. Follow the road around the town up to the next traffic light; you should see a supermarket just after the traffic light, on the right. Turn right at this traffic light, before the supermarket. Go straight up until the road crosses a river, and take the first right after the bridge. After 20 meters, you'll see a white building on the left: this is a Post Office. Immidiately after, on the right, is Sozankyo (with only a sign in Kanji). Stop the car in the courtyard, the staff will take over. Don't be alarmed by the external appearance of the building (from far, I found it looked like something resembling an agricultural processing plant), as inside is actually very nice. The garden is well kept too.
This appeared to be a very traditional ryokan, and I as we were the only non-Japanese there, I assume this was the real deal. As usual in ryokans, you have to remove your shoes when you enter, even to go to the reception. Staff are extremely enthusiastic and helpful (they even helped us send a parcel at the Post Office and set-up our Japanese Sat-Nav for visits around Aso). Their efficiency were only hindered by their relative lack of English, which led to some miscommunications as a result of which we missed our Kaiseki dinner, which was a shame as the food looked really good. The town has a few simple but good restaurants, so it wasn't a big deal after all. We did have breakfast, which was the traditional Japanese one, and very good and plentiful.
There are two segregated onsens (which we've not tried) and two private baths ("chartered bath"). The segregated onsens are also used as bathroom (to clean yourself), as the room only had toilets (at least the standard room we had). It is useful to know that there are washing facilities in the rooms the the private baths are in. Both private baths are actually really nice traditional ones, with a garden view and best of all are free (book at reception).
Our room was a standard guest room, non-smoking. It was situated on the first floor (no lift as far as I could see) and its layout was also traditional, with the usual small entrance, big tatami-covered room with a traditional table and a smaller room next to the windows, with a sink. The bed is made in the evening by placing a thin mattress and some covers on the tatamis. Even as westerner, we found it very comfortable to sleep on (obviously, you have to be able to raise from a floor position). The room had a portable combined A/C-heater which was more than adequate. The furniture was in wood and was inkeeping with the traditional decoration. There was no Wi-Fi nor any LAN available in the room but there was a free access computer at the reception. You're given a Yakuta and slippers for each person in the room, which is customary to wear in the ryokan (and useful for the baths); this contribute to the overall traditional feeling.
This was our first taste of a ryokan and we were blown away. We did try two others (one less fancy in Nikko and one more fancy near Fuji) and as traditional ryokans go (understand "proper" but not "modern luxury"), I still can't fault Sozankyo. I'll happily come back.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.