Hey,
Greenie, I had a 90 model 550 that I rode for around 12 years and I had an absolute blast with it!
Use this link for more research (if you haven't already); then I will provide some personal info:
The Zephyr-Zone - Index
They essentially used the old KZ motor in these, so they are as bulletproof as anything out there. The only 'problems' I can recall was a cam chain tensioner failure, and I had to replace the exhaust because the muffler cracked where it passed under the passenger footpeg. I did have a cylinder base gasket failure, but it was roughly 10 yrs old and 25K+ miles then; I doubt this was a common problem.
I'd guess availability is fairly low (for many reasons), but search the site linked above. This is the last one I've seen for sale in the last year or so (and it's pretty old to boot):
WTS/WTT 1990 Kawasaki Zephyr 550 - EastTNriders.com
I'd say performance is decent; the old air-cooled, 2-valve engines are never going to compete with a modern liquid-cooled, 4-valve design, but they are still plenty fast enough to get into trouble! Handling was decent, again no match for a true sportbike, but good enough I would regularly embarrass the squids in the corners.
Comfort is very subjective, but I'll say they are moderately to very comfortable. The 550 is/was a pretty small bike; it worked good for me (short legs), but if you're more tall /lanky than 'sqaut', I'll bet you would definitely want the 750 if not the 1100 model. The seat was fairly wide and flat; however, they had a slight slope on the front (where it meets the tank) that had a tendency to 'crowd the boys' against the tank, but it wasn't a significant problem to me. They also used a fairly flat handlebar and the reach is short. It puts you in a slight lean forward but never pressures your wrist like a sportbike lean. There's plenty of room to move around and adjust the bar/controls. I couldn't imagine comfort being any kind of issue.
The only downside to acquiring a bike like these are the typical issues of parts availability (tire choices are limited also), and the probable need for maintenance and/or reconditioning. The upside is a versatile, reliable,
fun motorcycle that is very unique. And drop-dead gorgeous IMO.
FWIW, I wanted to mention the original 550 Zephyr that Kawasaki imported was based on a 400cc model Japanese model. (Think the original Suzuki Bandit). I think this had a lot to do with my impressions of the bike (small but faster than any other 500/550's that were being sold then. I really think the Zephyrs and Bandits were ahead of their time for the US market, but in any case, they (supposedly) couldn't satisfy the need for the 'demand' of huge-horsepower, high-speed tastes of the American market. So...dealers didn't order many, demand was low, sales were poor, and it was downhill the rest of the way!
