A hell is basically a hot spring that you look at rather than get in, unless you feel like you could brace temperatures up to 100 degrees Celsius! I visited the most famous area, Beppu Jigoku Meguri.
You pay around 400 yen to enter one hell or 2000 yen for entry to all eight. I did the latter. There are red pools, blue pools, white pools, pools with bubbling brown mud and pools that spurt water. There is steam everywhere and bits of it do look quite hellish.
It was definitely an interesting place. In some ways it had a Disney feel or the feel of a 1970s theme park without the rides. One of my favorite hells was Tatsumaki Jigoku, the one that spurts boiling water and steam. I also liked Chinoike Jigoku, or as an English sign stated Bloody Hell Pond. Both of these hells were simple and were based around one basic pool. The restaurant at Chinoike Jigoku also did a great burger.
Some of the hells had animals you could look at, but for my liking they were in cages that were a bit small. It may have been the heat, but some of the animals almost looked a little lifeless and sad. I wouldn't let this put you off visiting though. It's a unique place.
There are other hells throughout the city and various hot springs that you can actually bathe in. If you're visiting Japan from overseas, you should go to a hot spring at least once.
I visited on a Tuesday, but as it was the last night of my trip, we headed into town to see what we could find in the way of nightlife. It turns out Beppu has quite a large foreign community, due to the International University within the city. We happened to stumble upon their summer leaving party in a bar called 68. There were probably people there from around 20 different countries. I think Beppu is the kind of place that could be fun and lively on a weekend...although I only visited on a Tuesday.