it is a fantastic lens, but there is room for improvement:
having independence in setting tilt and shift is really necessary, it should focus a bit closer, as currently there is a slight gap in fucus distance when using close up tubes and it eats more light due to its construction (when I switch between the tse and the 100 macro at closer distances there is a loss of light with the 90mm lens up to two stops). better fringing controll when tilted would be nice, but I don't know how much this can be improved with current sensors.
oh, make it f2, that would be lovely!