I use the 55mm Mamiya on my IQ160... it's okay, I prefer it more for the angle of view, but I feel a little let down by the performance... it's not bad, but it's not great either... at least in my case. I couldn't give a reason for it. Once its stopped down to f/4 it starts to perform well and very nicely around f/5.6-8.
My key fault is the sharpness fall off near the edges, and a little of the colour cast it appears to display... but these are in most cases overweighed by the other benefits of it.