I guess the ZRX is a "heavy" bike, but IMHO, that is a GOOD thing. It doesn't get tossed around at 80-90 on the freeway...and feels solid as a rock. I've had 10 hour days in the saddle and loved it.
Im not a big guy and never had a hard time throwing mine (ZRX1200) around. I just don't look at it like a roadrace bike, so most of the time didn't try to ride it like one. The engine is spectacular and it does everything else "good enough".
I've had an '82 GS1100E (not really a bargain anymore as you can buy a ZRX1200 for the same money) and they are fine. They kinda handle weird, but are stable (straight line) and quick.
and yeah.. I think the KZ650 is a much better bike than the 900 (Z1 and KZ900). I actually think Z1's are overrated, overvalued Big Fat Pigs. The only thing they have going for them is they are nice to look at....especially the '73 Rootbeer.
The KZ1000's are a better bike than the 900 and it would be a toss up which one I was going to hop on if I had a KZ650 and KZ1000 parked in the garage.
and then there's the Z1R-TC

or even the 750 Turbo.