Mostly hype in my opinion, but I still use one on my GT. It came with the Ford 'power upgrade' and I'm sure it does flow a bit better than the paper filter, though not to a degree I'm able to notice it. The trade off is likely slightly reduced filtration capability.
I'm thinking about buying an ecoboost MKT family car next year, and will definitely not use a K&N or any other 'high-flow' filter. I'd much rather have the best filtration possible for a turbocharged engine.
2011 GT Coupe Premium | MT82 | 3.55's | 400A | Red Candy/Stone | Security Pkg | Built 7/21
Installed: SVT Axelbacks w/ res delete, FRPP tune, K&N drop-in, strut tower brace, JLT oil catch can, PIAA/LEDs, GT500 splashguards, 27% tint
Sold: Rebuilt 1994 Mustang | Motorsports 5.8L/351W | HD 5M | DF clutch | 9" rear end 3.73s | Cobra 'R' brakes | aluminum driveshaft | street legal in SC (barely)