  We describe the performance of the Boolardy Engineering Test Array (BETA),
the prototype for the Australian Square Kilometre Array Pathfinder telescope
ASKAP. BETA is the first aperture synthesis radio telescope to use phased array
feed technology, giving it the ability to electronically form up to nine
dual-polarization beams. We report the methods developed for forming and
measuring the beams, and the adaptations that have been made to the traditional
calibration and imaging procedures in order to allow BETA to function as a
multi-beam aperture synthesis telescope. We describe the commissioning of the
instrument and present details of BETA's performance: sensitivity, beam
characteristics, polarimetric properties and image quality. We summarise the
astronomical science that it has produced and draw lessons from operating BETA
that will be relevant to the commissioning and operation of the final ASKAP
telescope.
