It's funny how peoples experiences vary depending on their location. The Z1 was definitely a CB750 killer here in Oz, helped by lots of race wins and a cult movie ("Stone") that pretty much killed CB750 sales overnight. Honda tried to build a "Supersports" bike around their SOHC 750 engine, and the CB750F was the result, which aesthetically at least, was no match for the Z1.
Sales of CB750's here were so poor that I was able to buy a shiny new 1975 CB750F fresh from the crate in the dealers warehouse in 1978, for a huge discount. (cheaper than a used Z1, which I would have preferred) I bought a K1 a couple of years later, and like Eric, the K1 is my favourite model.
Honda made an attempt to get a few more BHP out of the CB750 engine with the F2 and failed miserably, turning a good solid engine into a potential hand grenade, if not really well maintained. Cheers, Terry.