Coinciding with the launch of the completion suggester, we've seen a very large decrease in the zero results rate for prefix search http://discovery.wmflabs.org/metrics/#failure_breakdown! In fact, the zero results rate for prefix search is now lower than that for full text search. Similarly, we've seen a decrease in the overall zero results rate KPI http://discovery.wmflabs.org/metrics/#kpi_zero_results. Yay!
Dan