To date (2010) there are really two flavours, pre-2008 and 2008 onwards with one big difference.
Early (06/07) models have gutless low down caused by Kawasaki restricting them to make them easier for new riders, somehow having CG125 power up to 6,000 rpm, then immediately exploding into a ZX10R on steroids made them safer?
From 2008 Onwards this was fixed, but early bikes can be easily sorted if you remove secondary butterflies and fit a Power Commander, this costs a few hundred quid so be aware if looking at used bikes. Every owner I know who has done this rates it as the best mod they have ever done to any bike.
Some early ones were re-called due to frames cracking where the engine mounts had been overtightened in the factory, should all be sorted now but wirth checking if you have 06 / 07 or are planning to buy one.
Generally they are quite good out of the crate:
Later models or flyless early ones are mega grunty from 3,000rpm upwards, even two-up with luggage you rarely need to go above 6k or drop more than a gear or two for overtakes. Early ones with flies in are very flat and often need 3-4 downchanges when overtaking - very tiresome.
Handling is OK for a big bike, but can be made much better, they are better than ZZR1100's and such like but the front end is a bit harsh and the rear struggles with two people especially if you tour with luggage.
I had my forks and shock re-sprung and revalved and it became a different bike, cost about £500 but improved handling, feedback and comfort immensely. Felt about 20kg lighter afterwards, much closer to sportsbike handling.
Everything else is OK, clocks etc work well, comfort is pretty good, some serious mile munchers add bar risers, taller screens and Gel Seats but compared to any sportsbike it is a sofa.
Reliability is generally OK but there are some common issues, early bikes could leak oil from head or balancer shaft area, discs warp repeatedly, the Kawasaki ones are awful and can go in as little as 3,000 miles, I am on third set in under 10k - once warranty is up replace with aftermarket items.
Top fairings can crack around screws where lowers meet up as screws are so close the edge, look out if buying and be very careful when removing / refitting panels.
Tanks can rust under the seat, keep clean / dry and apply something to protect them.
I have been abroad quite a bit two-up with luggage, with the lowdown power sorted it was very good but suspension struggled first time on the roads of the Pyrenees, especially when loaded.
Second Euro-trip involved reworked suspension and the handling was excellent, bends could be enjoyed even with two people and full (hard Luggage) Stable wallow free handling with stacks of feedback - perfect.
Generally a very good bike let down a bit by a few niggles, but they are very easy to sort and can be massively improved quite cheaply making early ones a bargain.
Running costs are HUGE, Service every 3750 miles, minors are about £200 and Majors (every other) over £300 at stealerships.
OE tyres are poor, fronts feel vague and many owners get well under 2,000 miles from rear, I shredded two in under 3,000 steady (ish) mainly touring miles. Better rubber will see 3-6k depending on "Style", Roadsmarts and BT-021's are popular as all round tyres.
loads more info on www.zx14r.co.uk