I've used the 28mm on a H3D-39 since May and I shoot exclusively architecture. The lines are scary straight. I do an incredible amount of lasso outlining for my images and my photo style, the straightness has saved me big-time time. Prior to the Hasselblad, I did use the Phase One on a Rollei with tons of time spent for the infamous, uncorrectable, green-magenta shifts. Not with the 28mm on a Hassy. No plexi, just great color.
After a day running through the standard sharpness tests, I've stuck with F/8 and F/11 strictly with the 28mm. F/16 is almost acceptable, while F/22 is not. F/5.6 is not something for architecture, so I can not vouch for the 5.6.
The bad? Flare on the H3D is causing considerable retouch time. I've actually lost a few shots. Not a good situation at all, with questionable remedies.