Great park! I thought it was going to be a one hit wonder with just saguaros, but there is a surprising amount of different species of cacti and a massive range in wildlife from black bears, to bobcats, coyotes, javelina, foxes, rabbits, turtles, lizards, snakes, woodpeckers, road runners, and a ton more that you can learn about at either visitor center or on the information displays on any of the trails.
The west side seems to be better with a wider range of trails, larger visitor center, and more historical areas and picnic places. The East side is pretty quiet and flat, but better for biking and casual walking through the desert landscape.
We visited in February, and the weather was perfect. I imagine the summers are brutal.