Tires wear out every 2000 miles.
Gas mileage is terrible.
Handling is very weird.
Bike feels heavy on back roads.
Valve adjustment is very difficult for the DIY as the frame goes over the engine.
If you want to smoke all your friends on the highway, this is the bike to do it on. The power is overkill.
This bike is suitable for sport touring as it rides very smooth on the highway.
Applying full throttle makes the tire spin or the front wheel to come off the ground, which wears out the tires fast and also ruins your underwear. If you don't need to go 190 mph, then I suggest buying a different bike. It takes a lot of concentration to make this bike handle smoothly in the twisties.
I experienced no mechanical breakdown problems with this bike.
I have a 2001 ZX-12R and find that, like anything it is a compromise, however it is hyper-sensitive to tyre pressures - I run mine at 42psi front and rear which seems to work well, but anything below 35 and the bike wallows and is a real handful. Never ever had any problems keeping up with friends on any other bike, including the K5 GSX-R1000 and 06 Blade even riding on the famous mountain TT course (for those of you who don't know, outside of built-up areas, there are no speed limits at all on the Isle of Man).
Love it though and would not sell it for any other bike... well maybe the new ZZR-1400, but that is just an overgrown 12R in any case =)
I am surprised to read previous comments about the ZX12R being heavy on tyre wear. I have now toured Europe three times on my ZX12R. I run the bike on Avon Azaros and get about 7000 (yes that's thousand!) miles from the rear, and more from the front (I replace both together, so when the rear runs to the legal limit, I replace the front at the same time).
I am a big fat lardy git and my bike is fully laden when touring. I don't cruise around at low speed, but also I am not lifting the front wheel on each gear change.
The bike has a dreadful tank capacity and fuel economy, but it makes for one of the most comfortable 'solo' touring bikes I have ever ridden (22 hours in the saddle from England to Southern Spain non-stop except for petrol, a pee, un baguette y una cafe con leche.
The pillion seat is utterly useless, other than for storage!
So far the bike has proven to be an excellent all-rounder, a great bike for going out for a blast with some mates and a competent touring bike. Totally reliable and fault free to date!
I just purchased a mint 2000 ZX12R with only 2500 hundred miles. It has a powercommanderIII usb, titanium muzzy exhaust, and lowering links.
Nothing mechanically wrong.
Riding slow around town under 3,000 rpm proves difficult, especially in first gear; it feels boggish. The bike likes to jump if you're not watching your right hand at this rpm.
Lowering links were a bad idea. Although they make it look cool, wheelies often end in the tire being eaten due to dragging under the rear seat.
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