I guess it all depends on your method of powerleveling. I used ZF extensively when I was using RAF powerleveling, but then max level was still 70 and my mage was a little too squishy to try to pull the whole place on her at once. Smaller groups were not a problem, just slower (obviously). Plus, both my hunter and warrior had the extremely rare honor of getting both special swords to drop that combine into the epic two-hander (the name eludes me right now). Not really that uber once you get into outlands greens at 58, but it's a unique model with what looks like the Unholy enchant built into the model.

My progression was something like: deadmines 12-17, stockades 17-23, RFK 23-27, RFD 27-35, ZF 35-46, ST 46-54. After that, instance grinding was getting too slow for a 70 epic'd mage, so I knocked out 54-60 in just a few hours questing mostly in Felwood.

Plus, I just like ZF. One of the better designed original world instances, in my opinion.