I did 140 miles on the ZX4 today (a tankful, exactly!). First of all - OW!! I wasn't used the the cramped riding position, and by god, unfolding myself off it hurt! Second, my tyres are fucked, the rubber isn't heating up properly, leaving to some "oh fuck!" moments round roundabouts. I swear I lost both front and rear, and got a little drift on one!
Wasn't even going fast/leaning it much either!
Other than that, it's a great little thing. Top end's not as good as the Fazer, but the fairing gives better wind protection, and ducking down also means you get to enjoy the induction roar...