I really enjoyed ATM, but it took me a while to get comfortable with the fact that it was basically a conga line of tourists when you get to the dry section.
A lot of people say you need to be in good shape, but really you just need to be game to do some clamoring and be excited to try new things - it's not that physically demanding.
Reviewing the activity per se isn't really as helpful as reviewing tour companies would be - the guide you get is the key to getting the most out of your experience. We went with Hun Chi'ik, which seemed a little sketchy, but we all enjoyed our trip at the end (I was trying to listen to other guides on the way out of the dry section, however).
Another drawback is the price - it's not the cheapest experience you'll have in Belize, but it will definitely be a memorable one, so I would suggest taking the plunge.
Another warning: we expected this to take all day and didn't plan anything else, but if you leave by 9, you get finished in just enough time to tack on another short activity in San Ignacio (Cahal Pech, for example).
Overall a solid experience, but didn't reach the level of, say, Chechem Ha for me because of the hordes of tourists, camera prohibitions, cost, and average guide.