The greatest problem with medium format mirrors is that they tend to be quite heavy and cause a lot more vibration than the ones of 35 mm systems. Since film had a tendency to respond in a non-linear manner during exposure, images came out a lot sharper than they do with digital backs. Especially on classic systems like the V series from Hasselblad the rule of thumb now is to choose a shutter speed with a denominator more than three times the size of the focal length. So instead of the usual 1/100 sec. exposure one would use with a standard lens (80 mm) it is now more like 1/300 sec these days. This is also why there are so many 500 series photographers constantly using mirror lock-up.
Hasselblad's newer H system incorporates AF, which is nothing like fast and responsive DSLR AF, but one of the better medium format AF systems out there. The one certainty I know of is that they are losing market share and are quite desperate. Their entry-system H4D-31 has just recently been reduced to only £7.995 (see: http://www.hasselblad.co.uk/promotions/camera-prices.aspx
). Either they have a surprise up their sleeves for Photokina or it really is that bad.