Ok here it goes... but don't think I'm some sort of freak or something...
I checked this out a few years ago with an old university professor of mine. He told me that the theory behind this is accurate and there would theoretically be increased gas mileage... however... there likely wouldn't be a real noticeable difference because of all the other factors involved in engine performance and consumption. He told me you might see 1 or 2 MPG better but the rest of your engine would have to be in top shape (i.e. no deposits, always use good fuel, not stale, etc. etc.).
I've never tried one so I don't know how it would actually work in real life. Theory can be a lot different than real life as we all know. Personally... I wouldn't buy one.
Mike