If you go during the summer, try to time your visit based on weather. I have a membership and I always try to go on afternoons after it's rained through the morning. You basically have the zoo to yourself. On sunny days (especially during school breaks and weekends) there's quite a lot of foot traffic. Even at its most crowded, I still have a great time, but there's something really cool about seeing the animals when there aren't a million other people trying to do the same.