I'm a zoo lover and have been to around 25 zoos around the country. Would rate this in the top 10 for sure. Big zoo too, the only one i can think that is bigger is San Diego's. Nice exhibits in that the animals here have a ton of space, you definitely don't feel sorry for them as being couped up....they have room to roam and run. Their australian part was interesting in that the kangaroos and some other small animals roam free and you can walk amongst them. They don't attack, but like to watch you as you watch them. Very unique. Loved the polar bear exhibit, top notch. Boat ride across a lake and skyride in some ski-style chair lifts as well as customary train rides can help take a break from all the walking. Big big park so plan on at least 4 hrs here. We tried to rush through it and it still took 2.5 hrs to do. Exciting new Penguin exhibit coming in 2013.
Tip: Don't wear new shoes as you'll be doing a lot of walking.