I had one brand new in 1978 - one of first ones bought from Queens Park Motors in Manchester. Great bike and done about 30k on it trouble free (apart from snapped after market chain while touring Europe - cost £110 at German main dealers!) After previous Z1B, it handled great with lots of smooth power. Recommend one to anyone. I swapped it for a shaft drive GS1000G (bigger GS850) that didnt handle as well. Although not as charasmatic as Z1, was best bike I owned upto CB1100R I rebuilt in late 80s. Great memories!
I think the most sought after are early HC and EC and biggest issue is finding original exhausts. (Ive just sold one in bits a few months ago to concentrate on CB750 cafe racer project)
Phil