The concept is to provide a pulse from each side to aid in extraction & flow, similar to 180-degree headers. Placement, size, etc affect its efficiency, but if you've ever run a car with an H and then an X, what I noticed is the "smoother" exhaust note, and they make the RPM sound higher than it is. Everyone's "mileage" seems to vary with them, probably because of packaging/size/application variances; I'm not aware of anyone losing ET/MPH from one, but several people have claimed improvements. Hard to quantify though without a lot of data, and that's been hard to come by.
They do have an amusing side-effect though: even at only 6000 in the water box, most people will back away from my car, with concerned expressions on their faces; usually someone will come up to me later to ask if I'm turning 8,000 in the water and what my shift points are.