I've worked with the Hartblei 45mm Super Rotator and just got a Mamiya 50mm shift lens today. I've run a series of tests (taking two photos and stitching them in PS photomerge) with both lenses. The tests were conducted on a Mamiya 645 AFD/Imacon 384C, with the camera mounted on a Novoflex Castel-L rack to keep the lens(s) in the same position to prevent parrallax distortion. So, essentially, the camera and shift lens functions as a back shift on a view camera.
1. The Mamiya lens is sharper, both at the center and at the edges, more contrasty than the Hartblei, and the color is consistent with other Mamiya lenses, both AF and manual.
2. The Mamiya has a greater shift range -- 16mm vs. 12mm.
3. The Hartblei tilts whereas the Mamiya doesn't.
4. The Mamiya lens has a scale that shows how far the lens is shifted, whereas the Hartblei does not.
5. The Mamiya lens is lighter and easier to use.
6. It's very combersome using the tilt when taking a left and then a right shift photo for panoramas on the Hartblei, as you've got to completely rotate the lens to shift to the opposing side. The tilt has to be reversed also and to further complicate matters, the tilt knob does not have any markings. The Mamiya also requires rotation to shift to the opposing side, althought it does not have a tilt function.
7. The Hartblei is fine for single frame captures where PC and tilt focus adjustments are required.