While I've never done a geocache in a storm sewer, I think exploring them is really fun! Storm sewers are also relatively clean since they basically get flushed every time there is a significant rainfall.
Here are some pics from a storm sewer adventure with one of my friends:
http://www.mattkirkland.com/spelunk.html
For some reason, I always figured it would be against caching guidelines to put anything in a storm sewer. However, if it isn't, I would really love to place a storm sewer cache.