  We describe our completed spectroscopic survey for unbound hypervelocity
stars (HVSs) ejected from the Milky Way. Three new discoveries bring the total
number of unbound HVSs to 21. We place new constraints on the nature of HVSs
and on their distances using moderate resolution MMT spectroscopy. Half of the
HVSs are fast rotators; they are certain 2.5-4 Msun main sequence stars at 50 -
120 kpc distances. Correcting for stellar lifetime, our survey implies that
unbound 2.5-4 Msun stars are ejected from the Milky Way at a rate of 1.5e-6
/yr. The observed HVSs are likely ejected continuously over the past 200 Myr
and do not share a common flight time. The anisotropic spatial distribution of
HVSs on the sky remains puzzling. Southern hemisphere surveys like SkyMapper
will soon allow us to map the all-sky distribution of HVSs. Future proper
motion measurements with Hubble Space Telescope and Gaia will provide strong
constraints on origin. All existing observations are consistent with HVS
ejections from encounters with the massive black hole in the Galactic center.
