I had the pleasure of buying a brand new Z1-B in 1975. This was the last of the "really fast" ones, as the subsequent Z900 had modified carbs and airbox which made it quieter but took some of the ooomph away. Not a good move, as the opposition's bikes were getting quicker. Bulletproof engine, blistering acceleration, massive torque and supreme comfort (at sensible speeds).
The standard chassis/suspension set-up was totally inadequate for the performance, and high speeds (particularly on bends) led to soiled undergarments. Could be vastly improved by better fork springs, aftermarket shocks and steering damper.
Very fond memories of smooth effortless power.

The Kawasaki Z1 is steeped in modern-classic history and set the height of the 'bar' for others to follow. Late '70's and Suzuki blatantly copied the motor with their GS range. In its day the Kawasaki Z1 (900) was looked upon in awe by the motorcycle magazine testers. The engine was refined but very powerful, although the brakes at best ranged from 'marginal' in the dry, to 'non-existant' in the wet. The handling was barely acceptable, but not as bad as often made out. The machine does 'wallow' at motorway speeds, but it does let you know when you're reaching its limits. The styling is still stunning and this bike has an incredibly strong cult following.
Overall, the Zed is the most desired classic Japanese motorcycle ever.