I've been skeptical about these intakes as well but last night I went for a long drive to see if I notice differences and found that when driving slow it takes less pedal to keep it at speed so if anything should reduce fuel cost. Will keep an eye on mpg this week.
I wouldn't think it would drop power as it's breathing fresh air now rather than it's own gasses as well, certainly gets up to speed the same just with added noise =]