I had a K&N filter in my 2011 Fiesta for about 90 days until I got a cold air intake for it that came with a dry flow filter. The only "problem" that I'm aware of with K&N filter is when people OVER OIL them and the oil gets onto the MAF and ruins it. Other than that they are fine. I got a Green filter from FSWERKS for my Fiesta ST which is similar to K&N.
K&N filters (as well as the Green brand) do flow better. At the end of the day you may pick up 2HP with this filter. The problem with them is that they let in a lot more dirt. if you plan on keeping your car for a long time they are not the thing to get.