Never ridden an FZS600, so wouldn't comment on one. However, have owned an FZ6 S2 fazer for 5 years. Used to for commuting to work and back for 2 years before I retired, proved to be reliable, never failed, stood up to shit weather well with no signs of rust or corrosion (but I did acf50 protect spring and summer). Road holding was good during winter with decent tyres on it (bridgestone BT023), no oil usage and good economy. The fairing also provides decent protection from the elements. I have added a lower fairing to mine for even better protection.
The bike is nimble and turns easily, and is not expensive to maintain, provided you carry out some form of maintenance regime. Seat height is fine is your short like me (5ft 6") although I did lower both the front and rear slightly so as to be on the balls of the feet rather than the toes.
Front suspension is non adjustable, rear is adjustable for preload only.
I have found the seat comfortable for longer journeys and the bars are positioned at a height that does not put pressure on the wrists, and keeps the back a little bent so as to avoid pressure up through the spine
I found the clutch hard work in traffic, going up and down the box, and converted my to hydraulic for much easier use. That has been a revelation. The gearbox is a little cluncky form neutral into first, and from third down into second, but they're all like that apparently
The brakes really are very good. I can outbrake many other bikes on the track with them, they are strong with no fade, but that could alter depending on pads etc. I am currently on EBC FA252HH sintered. The really brake is almost non existent. Your better off taking a deep breath and blowing out in front off you.
For a bog standard road bike, performance is very good, moves very quickly once you get to 8000 rpm and ive never had any vibrations.
I think in value for money terms, they are good value at the moment, more modern bikes like the MT07/ninja 650 etc sre down on power and performance compared to the FZ6 S2. There are modifications available too should you wish to go that way. After market exhuast will remove several kilos of flab, as will removing the cat convertor
Shop around, there are plenty to choose form. Check the mileage, valve service is at 24,000 so be aware of that
Hope this helps. They are seriously good machines