Is it worth paying the $50 for a K&N replacement filter or just the stock one. I'm not looking at spending $200+ on the big K&N ones.
I have a 4.6 V8 F-150

 

If you are going to keep your truck a while, then yes. I was replacing the paper filter yearly (about 18,000 miles). The K&N seem to last a little longer between services. You will eventually need to buy a cleaning/reoiling kit for about $11.00 (and good for about 4 cleanings). Just make sure you don't over-oil the element when you do clean it. Some people have had some minor problems with their MAF fouling, but that's probably because they over-oiled the filter when they cleaned it (plus you can usually clean the MAF anyway). I figure mine paid for itself in about 4 years. I also seem to get about 1/2 mpg better gas milage.