On the roof of a hotel I stayed at in Japan they had a posh onsen. The building was the tallest in the town, so you could see the twinkling skyline at night. In the two days or so of staying in the little town, I must have visted the onsen 4-5 times, I loved it! When I returned to the U.S and told my friends that I went to a public bath, they gave me "The Eyebrows". (aka: Where is the Hannah we know and love? Who are you working for, ye perv!) (Ok, not quite like that.)
I can understand why people immdiatly jump to the conclusion that an onsen is a perverse and immoral pastime with sexual connotations, but it is not. Onsens are places for relaxation and friendly conversation, for meeting up with your friends and talking about whats up in your fishbowl.
I would highly recommend going to an onsen to anyone travelling in Japan. It is a bit uncomfortable at first, yet not a waste going. Its sad that people are so nervous or misinformed that they pass up the chance to experience a memorable and plesant experience of an onsen.
Here is a map of Japan showing some of the more famous onsen spots in Japan (there are many more):