On my recent visit to Maui, my girlfriend and I booked a few different tours/activities offered from TripAdvisor and other services, and by far Lahaina ATV Adventure was absolutely the best one of all! We really enjoyed driving the ATV's through the bumpy dirt terrains on the remains of an old pineapple farm and making stops along the way to appreciate the amazing view and the nature of the island. Also really enjoyed the company of our guides Alan (Allen?) and Tiffany. They were very friendly and pleasant. Fun people to talk to. They also shared some good nuggets of knowledge and recommendations about the area. The only regret I have is that I didn't have enough cash to tip them at the end.
As for the activity goes, it was a bumpy ride but nothing too extreme. I was a little concerned at first since my girlfriend gets sore back and neck easily, but she was absolutely fine. The dirt/dust will get on everywhere and get in everything you take with you. It is a very dusty experience, but not in any unpleasant way. We even had bandanas around our face inside the helmets to cover our noses and mouths, but dirt still came up from our nostrils and even ears when we got off the tour. Face coverings recommended & don't wear your good clothes if you don't want them dirty.