These slides are from a talk that Tyler Tate presented at the Enterprise Search London meetup on February 4, 2010.
Careful observation seems to reveal three different approaches to search suggestions: completion, suggestion, and instant results. These approaches range in cognitive burden on the one hand, and utility on the other.
Completion is easy to understand and almost always results in a net benefit for the user. Suggestion, because it throws new ideas in the mix, is slightly more complex, but that is usually outweighed by its helpfulness. And finally, instant results work well for simple, highly structured data, but at times runs the risk of overwhelming the user.