Our family has visited a handful of zoos over the past few years and the Jacksonville Zoo definitely competes with the zoos of some of the larger cities. I found this zoo to be more enjoyable than the Atlanta Zoo and on par with, if not better than Tampa's Lowry Park Zoo.
The areas and enclosures for the animals appear to be expansive with good upkeep. The exhibits are fun and some interactive (like the stingray touch-tanks and giraffe feeding area).
One thing I really liked about this zoo is the size of it and how the exhibits are laid out. Every corner seems to lead to a new path/exhibit. As we toured, we didn't look at the map just to surprise ourselves with what's around the corner. Towards the end, we found the Japanese Gardens, which was a beautiful exhibit that the wife really enjoyed walking through.
A couple suggestions: This is a large zoo, wear good walking shoes. Also, be prepared to sweat if the temps are above 85. If you don't like the heat (like I don't) then either go in the early morning when it first opens (also for smaller crowds) or go during the fall/winter. Also, the restaurant that looks into the jaguar exhibit can get crowded during busy lunch hours. It can also get busy and loud if the jaguars come to the windows in the restaurant, which typically results in a lot of eager on lookers trying to get a better view.
I plan on visiting this zoo again on a more regular basis with the kids - but will avoid going during the summer months.