I've found TheKnot to be the most useful to me by miles. Not so much the core site, but it's been very valuable to read through the forums, both the general photography forums as well as the geographically local ones. It's been great to get an unfiltered view of what brides think, what they liked about this provider but hated about that, how they're dealing with budgeting, the decision making process they go through, etc.; it's all there.
Although there are MANY wedding photographers across northern NJ, there are really only a few out in my more remote and rural western county so those guys are my most direct competition. It's especially fascinating to hear what brides have to say about these guys after meeting them. Even more fascinating, of course, is what they have to say about YOU after a meeting. These insights have really helped me shape and refine my approach during those initial consultations.