The autofocus or manual? I have used the manual and the image quality was good. I also liked its ergonomics. I know the AF does get good reviews in comparison to others.
I don't know if this matters to you, but to me it does. Medium format lenses are so expensive that other then the normal lens many people rent. In my area that means you need to have a Mamiya or Hasselblad.
This is a little off topic, but not much. If you look at magazine reviews for medium format notice that most everything is universally praised. I only mention this to say I think you are smart to find out about it this way.
As Guy mentioned, it could be worthwhile for you to take a look at the Mamiyas that have now an over 50 year tradition. Mamiya began making the Mamiya 6 rangefinder and later their famed Mamiya 7. Today they offer a host of models, either manual, auto and with optional finders and winders. The compact 645E model is a relatively 'economical' cost-effective entry into the world of medium format (for under US$1,000), basically because its film back is fixed, i.e. not removable. Nevertheless, it has a meter for aperture priority AE. It also has AE compensation in 1/3 of f/stop increments, multiple exposure capability and mirror lockup. Available Mamiya Sekor lenses range from 24mm to 500mm in focal length. In contrast, the Pentax 645NII, which has also gotten very good reviews, costs more than twice as much.