Surprise can be very effective. An element of surprise will help make your message more "sticky", as the Heath brothers point out in their book; it's the 'U', for "Unexpected", in their SUCCESs formula. If you think back to presentations you attended and what you remember best, you'll often find a memory that's attached to such an element of surprise - a surprising fact, statement, or something unexpected the presenter did. Or, sometimes, just an unexpected accident that happened during a presentation.
So how, exactly, does surprise work?