It used to be a Google search box and it worked even worse than it does now. As far as I know the search is currently handled via Sphinx so there's plenty of options for tuning it, for example I think it'd be a lot better if it was configured to do partial word matching on the title and give that more weight than the body.

Experimentation should lead to much improved results.

The biggest problem, though, is that when searching in the search box at the top of the wiki you're taken to the search page but not shown any results.