I've found Zagat online to be completely inadequate. They don't even have full listings for all cities, I've found. I tested that on Houston, for example, and they only had about 80 restaurants for the whole city listed. That doesn't make sense since there are way more in the book ...

On the whole, I think you have to take any reviews with a grain of salt. Food is like wine -- you'll develop your own taste for it. But by reading lots of reviews and getting as much information as possible and seeing which places sound like they'll mesh with your taste. You can glean a lot about whether or not you'll like a restaurant by reading between the lines a little bit and comparing notes from various sources.