The Contax 645 works better and integrates better than Hasselblad H1, H2, 500CM, and Flexbody with Hasselblad CF-22 and CF-39.
When the same backs work with H1 or H2, I frequently encounter start-up errors, which needs to recycle the power. I've never have any problem when the backs work with Contax 645.
The Contax 645 has a long reputation of battery hungry, but interestingly, the Hasselblad H requires much higher power from the firewire, it does not work well with the current iMac which has only 8 watt from the firewire ports, it requires the firewire port of 12 watts, but Contax 645 has no such problem.