Also depends on the areas that you live in - temperatures, type of dirt, size of particles, etc. An oiled-gauze type air filter may be good enough to stop the majority of the contaminants that it shouldn't affect lifespan of the vehicle that much. Like they say - your mileage may vary.
K&N setup is good enough for TRD to use them as the vendor for TRD badged performance air filters (looks exactly like a K&M filter, just different colors).
I've been running a K&N on my 2002 Corolla once the powertrain warranty ran out (60K miles) - have little over 205K miles on it, with 145K of those miles on a K&N filter. Pull UOA (used oil analysis) annually to check on the condition of the car - as I run extended oil changes. Both wear levels and amount of silicon contamination (usually a sign of sucking in more dirt) - were pretty much the same between running the OEM paper vs the K&N aftermarket.
Much of the issues I've notice on the boards on running these K&N is likely due to improper oiling techniques to re-establish that oil "tack" behavior. I clean my K&N once a year (remove, wash, dry, re-oil, replace) and re-oil every 6-months. There is a trick to making sure that all the pleats get the right amount of oil.
I only like them because I can run one filter for basically the life of the car. Cost and performance wise - they are pretty much the same. What you save in number of paper air filters - you'll eat up with maintenance on the cotton gauze one. K&N is not as acoustically "quiet" as paper - so you'll hear more of the intake noise compared to the OEM one, but MPG, acceleration - butt-dyno said no difference.