I think one needs to use Zagat or any other guide for what they are -- guides. They are not the "be all or end all" of good food. However, they can be helpful in showing what an aggregate opinion of a certain place may be.

More than Zagat or other guides, I tend to trust the Hotel concierge for their recommendations (of course staying at a top hotel helps but is not necessary). Otherwise, I try to find local food/restaurant guides more designed for locals for guidance. For example, here in London, I find Zagat not to be terribly useful and slightly pretentious. The local guide, Harden's is much more informative and covers a lot more restaurants than Zagat does.

That being said, I often disagree with many guides -- Michelin, Gault-Millau, Harden's and Zagat included. Perhaps, that is part of the fun; to discover these things and make your own notations.

Happy Eating!!