There's been lots of discussion about this--search is your friend.
To answer in short: the stock filter is cylindrical, the K&N replacement is part #E-2993, and the consensus is that it improves hesitation and drive-ability.
Hatch Nation #90
'12 SE Sport 5Dr |╠╬╝|Sterling Grey Metallic|203a|Moonroof|17" Paint/Mach Whls|K&N 57S-4000|FFE RMM|Steeda STB, RSB|JBR bushings
'01 ZX3|╠╬╝|Pitch Black|Prem. pkg.|Moonroof|SVT Snorkel|SVT Springs|S2 Dampers|Steeda STS|Magnaflow exh.