I used a Rollei 6003 for many years back in the 1990's. I worked for a natural resource firm and photographed, from the air, forest land found in northern Michigan. We used a 6003, a 70mm back, and several hundred feet of Kodak CIR film (infrared). The plane we used, a Cessna, had a hole cut in the belly pan so the camera would point straight town. A nice setup.
The only drawback to the camera was its batteries. The NiCads we used were not at all reliable (because of the memory effect) and we eventually jury-rigged something that would allow us to draw current from the plane. Other than that, the 6003 was/is a great camera. I wouldn't hesitate to buy one used, if I was in the market for a medium format camera.
Rollei now offers Nickel Metal Hydride batteries for the 6000 series cameras. NiMh are much better than the old NiCads. No memory effect. If you do buy 6003, I suggest dumping the NiCad batteries and going to the NiMh variety.
Hope this helps.
Jim B.