Beautiful place. However, I agree with another review that $18 to look at cacti seems a bit steep.
Wear your walking shoes and be prepared for a lot of uneven surfaces and trails as well as extreme walking. Wear sunblock since there is very little shade and bring drinking water since there are not a lot of water fountains along the way.
Bottom line: A great way to get lots of walking exercise, sunshine and fresh air but $18 is a little pricey.