It depends on the kit. Change of driving style aside, if you go for an open filter, you will most likely see a but more fuel consumption, and it won't give you much of a performance gain (if anything it may decrease it).
Enclosed filters are far better, as they stay cooler, and therefore so does the air going into the engine. For this reason, many people have the opinion that the standard induction system on the car will be much better than an aftermarket open filter.
Of course, go ahead and get one if you just fancy a bit more noise
Just don't believe everything you read in the sales descriptions lol.
There are loads of threads on this. Have a search.