  The visitor instrument PIONIER provides VLTI with improved imaging
capabilities and sensitivity. The instrument started routinely delivering
scientific data in November 2010, that is less than 12 months after being
approved by the ESO Science and Technical Committee. We recall the challenges
that had to be tackled to design, built and commission PIONIER. We summarize
the typical performances and some astrophysical results obtained so far. We
conclude this paper by summarizing lessons learned.
