I run a stock replacement flat K&N filter. I have not had any problems with it over the near 240K miles it has been on there. I can't say it improves performance or gas mileage, but I like it because I can clean it. I have run K&N filters since the mid 80s and try to put them on all vehicles.
Many people say they pass dirt and are bad for the engine or they contaminate the MAF. I have never experienced any of this. If they are used correctly, they are good.

 

I own a 2005 Ranger and have owned it since new. I installed a K & N filet a week after purchasing the truck. I modified the air flow box to give it more air with no problem. Clean your filter according with K&N instructions. Do not over oil your filter !!!! It will give your mass air sensor problems!!!