i ride an '01 and it has been everything i need so far. i got it out of an insurance lot for $2200 2 years ago and have spent some time learning about it and getting it back in order. i use it as a courier in a financial office as well as recreationally. they look bigger than new bikes are, but they as quick as you could want and have a relatively smooth powerband. my only complaints with the engine are no power below 4000 (same as all 600s i think) and between 8100 and 8500, there is a serious power dip (don't know if this is common, though). otherwise, it has held up well to a beginner learning to ride and having a blast on it.
a big plus is to it is that it's identical to the new zzr600, so kawasaki is still making parts for it and will continue to for a while.