I own 90 and 180 mm KL and 50 mm C lenses. I find these give medium contrast and more than adequate resolution. I have never had occasion to complain about flatness of field, but then I've never shot pictures of lens charts at full aperture. The main thing that bugs me is the clear barrel distortion with the 50 mm. Compared with Mamiya TLR, which is what I had before, I'd say the image quality is much the same, the 50 mm RB lens is much better than the 55 mm TLR lens, which I found needed a lot of stopping down to bring the edges in and was prone to flare (I tried 2 examples, both the same).
Compared with Hasselblad, I'd say Hasselblad Zeiss lenses have slightly higher micro contrast (more "bite") and the w/a lenses are free of distortion (as they should be for their eye-watering price). I have been more than satisfied with my RB outfit, as far as I am concerned the only worthwhile trade-up would be to Hasselblad, but I like the 6x7 cm format and don't necessarily feel the Zeiss lenses are worth 4 times the price of Mamiya.
Regards,
David