The ZZR600 is an extremely underrated bike. I have had one for about two and half years now after upgrading from a GPZ600R which had served me well. For the price you get such a lot of bike for your money.
Most articles on the ZZR600 will dismiss it as too old to party and far behind todays tackle - it hasn't changed much since 1991 apart from a mid nineties facelift from the D model to the E - also know as ZX600D or ZX600E. Its true that if you want a track focused bike or something just to go mental on Sunday with, you'd be better looking at something more modern. However, if you want a bike you can use every day of the week, commute on and do track days and Sunday blasts on the ZZR is a good choice.
Its a very comfortable bike and is such a nice place to be I have regurlarly done 600 miles days on it touring. Racking up the miles increases the confidence and lean angles, and you'd be amazed the amount of R1s and Fireblades I overtake on the twisties, as the riders just havn't been out on their bikes as much and don't dare push as hard.
As its been around for so long its really cheap to run, most parts are available on ebay all the time, though regulars like oil filters, cables etc I've often found cheaper at the Kawaski dealers than what people are bidding for second hand ones - so its worth checking.
Reliability is brilliant. I've done over 35000+ miles in two and a bit years now and the only thing thats gone wrong was a snapped clutch cable. Even with the broken clutch cable I managed to bang it into gear and clutchless change 200 miles home - it'll start in 1st gear and ride off with no clutch on level ground - amazing!
Obviously if you buying one of these check the usual things like blue exhaust smoke, water in the sump, overheating, and make sure things like head bearings and wheel bearings are sound. Exhaust down pipes will eventually rot through if you use it in winter - nothing else seems to corrode though. I'd recommend picking up a bargain stainless steel downpipes off ebay if you see one, as eventually you'll need it. My originial pipes rusted through after the second winter, when the bike was 6 year old, but the new stainless system off ebay will last forever I reckon - and its so simple to change yourself. In fact its a very easy bike to work on generally - plenty of space to get at stuff, simple technology thats easy to work out, the center stand provides instant lift to lube chains and change wheels etc.
Overall, if you want a good bike that will do just about everything at a bargain price and be cheap to run grab a ZZR600.