Jigokudani is obviously a well known attraction in Japan, but the photos in this blog are deceiving. Although not obvious from the left picture, the one on the right is clearly a photo of the Womb ... note the rock formations. This very much illustrates what goes on there during the darker months :-)
http://www.wa-pedia.com/japan-guide/nagano_obuse_yudanaka.shtml
Seriously though, I would think the colder months would be most inviting for a visit, but heard the high water levels may bury the pools.
note: using new username (mxa)