I've been on one.
It does totally depend on the place you go, lots of places do have free passes etc, but some sites you can end up paying a lot more. We went to Hendra in Newquay and we did have to pay for passes and stuff but it was worth it.
I'd say pick your place carefully and they can be great, but also look at last minute offers at the parks cos they can be just as good if not better.