Gorgeous! Very worth a trip here. One of the best attractions we've seen in a month of wonderful sights and experiences.
Some of my tips are repeats from other people but in the interest of keeping things up to date, I'll add them anyway. Call ahead if you think you're going to get there toward the end of the day to make sure you can still go that day. Buy tickets online and save a couple bucks.
If possible, try to get on a tour without small children (unless you have small children) - it's hard to watch out for them when you're taking pictures and looking around. Also, the acoustics are such that their sweet, loud little voices make it hard to hear the guide.
If the guide isn't waiting for you to take your pictures, ask him/her to wait or turn the lights back on. It's hot and very humid in there. You can't take water, so drink up before you go down.
In some areas you'll need to watch your head. My son bonked his pretty hard. It bled and he now has a bump for a souvenir.