I have been 2 or 3 times.....each trip with my Scout troop back in high school. It's neat to go if you've never been. But I don't really care much for spelunking, so I won't give you a raving review. If you've never been in a cave before, it's pretty cool. It's open enough to not cause claustrophobia for just about most normal folks.
1) Take ear plugs if you're doing the sleepover portion of it. Imagine about 200 kids of various groups trying to go to sleep at night
2) Take junk clothes you won't miss by throwing away. "Bubble Gum Alley" will ruin every thread on your body.
3) Buy a head lamp. I can't remember if they provide them or not, but you won't have an easy time trying to tote a Mag Light.