IMO they suck. Sound great but suck. Most can even reduce performance because they are not taking in clean air as they are not encased. As stated above the Z ecu does adapt and cancels out any potential gains.
Mate of mine has a Skyline 34. It arrived in the country with a charger installed. I got him to source an original air box. Power increase, drive was smoother, oil consumption returned to normal. Happy camper.