I didn't have time to visit all the hells when I was there a couple of weeks ago, but I've got a few pics that should give you a fair idea of the place.
Kamado Jigoku ("Cooking Pot Hell") "features several boiling ponds and a flashy demon statue as cook."
On this particular trip we also visited Oniyama Jigoku ("Monster Mountain Hell"). This picture I posted a couple of days ago should give you an idea of what to expect there. Other "hells" are "The White Pond Hell," "Golden Dragon Hell," "Blood Pond Hell," and "Spout Hell."
All in all, The Hells of Beppu are a nice blend of tackiness and geology. Who could ask for more?