The Multiple Mirror Telescope (MMT) adaptive optics system (MMT-AO) has been operated in a campaign mode for the last two years. In total seven runs, each lasting about two weeks, have been carried out. During these observational runs a large amount of data have been collected. These data allow us to draw some preliminary conclusions about the overall system performances. In this paper we discuss in detail the achieved performances of the MMT-AO system which is equipped with the first adaptive secondary ever developed. The performances are examined both in terms of number of corrected modes and control bandwidth achieved. We also discuss our attempts to improve the system calibration. This is done by modulating the internal slope offsets while the system is operating in closed loop on the sky.