Not having a go at the K&N fans, but don't always believe the marketing and be sceptical. If the K&N has a better design and construction, better filter element, etc. then that's all great. If they add a magnet and a nut then charge extra $30 for essentially the same thing (or even inferior), is it really worth it?
Not sure if you lot can see this, but on the R31 Skyline club, someone did a comparison of the Ryco and Nissan oil filter. Would be interesting to see the internals of the K&N and other filters.
http://forum.r31skylineclub.com/index.php?topic=103539.msg975257#msg975257
*edit* ahh replies, cheers fellas.