My wife and daughter took this tour today--what an experience! I would highly recommend the tour for any moderately-adventurous travelers. Our guide, Ben, was fantastic. Very knowledgeable about the surrounding area, but most of all, very competent and made the tour safe for all.
The ATVs we drove were modern, well-maintained and came with all safety equipment. Each group (of up to four people) rode in the own vehicles, while the guide led the way.
The terrain was a mix of rocky trails, sandy areas, and well-marked paths over hilly rock outcroppings. The speeds through the trails and tour were very manageable--in many places no more than 10-15 mph. Some of the uphill and downhill paths were a little steep, but the ATVs had no problem navigating. (All drivers must be 21 or older, though the trip would be suitable for children--perhaps as young as 6-7.) This is a 4 x 4 experience, and the ride is bumpy in places. If you're of poor health or have back problems, you may want to think twice about it.
We made several stops along the 2.5 hour trip, while the guide explained the history of the area, explained the geography and wildlife in the area. The overlook of the Colorado River was the turnaround, and probably the highlight of the trip (though the driving was a blast, too), and there is plenty of time for photos. Ben, our guide, had plenty of water, and you could probably bring light snacks--though you will need a free hand to hold onto your seat!