The interesting thing about Hakia, in this case, is that the vertical search engine is integrated into regular search. They’ve done a good job of making the engine automatically direct you into a health search, if that’s what you’re looking for. So rather than trying to sell a unique vertical, they’re adding functionality to their existing, whole-web engine.

That said, you’re right; it’s always going to be difficult to convince users to switch unless the benefits are blindingly obvious. In this case, I’m not sure they are.

From my experience, convincing users to switch to you because of some subset of searches is a difficult value proposition. Most users don’t think in terms of verticals and can’t remember which engines to use for what task: that’s why most people start off with a Google search. Engines like Trulia and Kayak do well because they search a different dataset that’s not accessible via text (but even they rely on a large part of their traffic from Google).