The best search engine in the world cannot convert garbage into useful content.
So, before you spend megabucks on the latest cool search tool, think about what repository you're searching. If your office is anything like most offices, you've got tons of ephemera -- stuff that probably isn't going to matter 30 minutes from now -- and tons of materials that really ought not to see the light of day. However, a decent search engine has the effect of shining a klieg light on the shortcomings of your content.
In fairness, a great search engine can help filter out much of the suboptimal content, but if there isn't anything worth finding, what have you really achieved?
One solution is to stack the deck: make sure you put a goodly amount of useful content into your repository before you turn the search engine on. And what constitutes "useful content"? Not just something that someone might possibly need someday. (That's the sort of rationale the turns a document repository into a cluttered and confusing packrat hell.) "Useful content" is content that is vetted: someone has reviewed it and given it a seal of approval or it's been used with good results enough times for you to know it's a great resource.
With that kind of content, you're going to look like a genius every time the search engine does its thing. How often can you say that about a KM project?
For more information on the benefits of vetted content, see my article "Loading the Deck" in ILTA's 2007 Knowledge Management White Paper.