Good link. I don't think it's an "either or" question with vertical vs. horizontal. Users are going to use a horizontal search engine when they want a general overview of something or don't think they need an expert's-eye view of a topic (maybe they're just getting interested in a new hobby and want to take a general look at what's out there). For something that's more technical or where they themselves have a lot of knowledge in the area, they'll probably at least try a vertical search engine if they know one exists. Of course, that's just my opinion; your mileage may vary.