Chester Zoo was fucking ace. No point in trying to compare it to Dudley Zoo, as it's a bit like comparing Sergio Aguero to Frank Nouble.
We got there for 10am when it opens and didn't leave til gone 5, yet we still didn't get round the whole thing (though we did have 2 small children with us who don't walk quite as fast!). The exhibits are fantastic - you can tell how much hard work has gone into making their environments as suitable, safe and as close to their natural habitats as possible. The Jaguar pen in particular is awesome (though it was a bit like the start of Jurassic Park as we couldn't see the fuckers - fortunately I went back later on and saw one of them).
We got amazing views of so many animals - elephants, black/white rhinos, chimps, mandrills, orangutans, komodo dragons, lions, tigers, bears, penguins, butterflies, pelicans, otters etc etc etc. Tons of stuff to see.
A highlight was the fruit bat forest - basically a very dark enclosure with loads of trees and bits of rock with huge fruit bats hanging all over the place, and they swoop down around you. I got 'whooshed' a few times. Loved it. They're big buggers too.
It's not the cheapest place in the world as you might imagine but if you're able to go there for a full day, make sure you do because it was a brilliant day. The kids loved it and there's stacks to see in one of the best environments I've experienced - certainly in the UK.