It's a popular class that is traditionally dominated by Honda's CBR600F, Britain's top-selling 600 in no fewer than six of the past seven years. This year Honda has again revamped the CBR in an attempt to defend its position against attack from Kawasaki's latest challenger, the ZX-6R.
Like most rivals in the price-sensitive 600cc market the CBR and ZX-6R are very similar in specification, each having a watercooled, 16-valve four-cylinder engine producing about 100 horsepower. One significant difference is that while the Kawasaki's protective fairing is shaped to reveal an aluminum beam frame, the Honda relies on a steel frame, hidden behind more comprehensive bodywork.
Although it is far from being an all-new model like the ZX-6R, the CBR600 incorporates far more modifications than its almost unchanged styling suggests. From the rider's seat the most obvious difference is that this bike's screen is more racily angled - to improve aerodynamic efficiency, according to Honda. Ironically, I found the lower screen a disadvantage, because at speed more air is directed straight on to a tall rider's chest.
The CBR certainly zips along quickly, thanks partly to a new "ram-air" induction system, which uses the bike's movement to provide a slight turbocharging effect. Performance is improved a little, particularly at higher speeds without compromising the Honda's smooth and refined behaviour at low revs. Never mind that this bike is "only a 600". By any normal standards it's thrillingly rapid, with enough acceleration away from the lights to make a typical Ferrari driver's face match his paintwork.
The Honda's chassis, updated this year with revised suspension, larger front brake discs and a wider rear wheel, is even more impressive than the motor. On the road it is almost impossible to make the CBR misbehave. Its steering was light yet stable, its brakes powerful, its suspension firm yet compliant. The CBR has traditionally been the yardstick by which other 600s are judged, and this model raises the standard still higher.
But the Honda is by no means perfect. Its fuel tank is slightly larger than before, but the range is still only a little over 100 miles. Although the CBR is reasonably roomy and has a comfortable dual-seat, I found the fairly upright riding position too exposed for long-distance riding. And although the Honda's styling is neat and the standard of finish excellent, after four years the CBR's looks hardly set the pulse racing.
In contrast, Kawasaki's designers started with a clean sheet of paper, and the 7X6R benefits from their apparent determination to make this the sportiest and quickest 600 of all. Although the new bike resembles other Kawasaki models, its aggressive styling just about manages to make the ZX stand out from the lookalike 600cc pack. And this is certainly the fastest bike in its class in a straight line, with a top speed approaching l60mph.
That speed comes from a four-cylinder motor developed from that of Kawasaki's powerful but heavier and less sporty ZZ-R600. Kawasaki was the firm that pioneered ram-air induction on motorcycles and, like the CBR, the ZX-6R uses the system to produce stunning performance approaching the 14,000rpm redline. Even in cold and damp winter weather the Kawasaki rarely failed to excite.
Its slightly peaky power delivery encouraged frequent use of a six-speed gearbox that was a little more precise than the CBR's.
Having had the opportunity to take direct aim at the CBR, Kawasaki's designers produced a chassis with near-identical dimensions. Cycle parts are also very similar, particularly the multi-adjustable front forks and rear suspension unit; the Kawasaki handled, stopped and gripped the road superbly. Maybe the ZX felt a fraction more agile and the CBR marginally more stable, but there was really nothing in it.
For a sports bike the ZX-6R proved reasonably practical, too. It is slightly roomier than the CBR and has a taller screen. Low-rev response was not quite as smooth as that of the impeccably refined Honda, but the ZX-6R was docile and easy to ride in traffic.
Unlike the Honda the Kawasaki has no main stand (making rear chain maintenance inconvenient), but it matches its rival with comprehensive instruments, clear mirrors, a broad seat and solid handholds for a pillion.
These two bikes are so closely matched that anyone looking for a fast, fine-handling middleweight would doubtless be delighted with either. At £6,595 Kawasaki's newcomer is the more expensive by £70. But despite that, the ZX-6R's fresher looks, better wind-protection and racier character just about give it the edge.