The Focus comes with a "lifetime" air filter system. Fucking PZEV vehicles. This is what the stock system looks like:



No replaceable drop in filter, so a CAI kit gives you that option. Not to mention it is like $400 to replace it all.

I replaced mine with this:



3" tubing, insert for no tuning. I see more MPG and peppier acceleration. With my tune in and the insert removed I may see 10 hp to the wheels. But I can clean my filter and inspect it without dismantling my front end.