Staying in a ryokan is an excellent way to get a traditional Japanese experience. Id recommend doing it in Kyoto for a couple of nights at most. As an earlier poster mentioned they do vary widely in comfort level but no matter what you will be sleeping on a futon on top of tatami, so it will be harder than your mattress at home! Its still a great unique experience, especially when you get your meals served to you in the room.

Also, the earlier post saying that they are only open to Japanese is not correct. I am a caucasian american and have stayed at a few of these hotels over the years.