I listened to the comparative sound clips on the Bill Lewington O-Port web page - and could detect very little difference between "without" and "with".

I also don't like the way the device is stuck into the rim of the soundhole - it looks as though the natural vibration of the face will be inhibited. I've asked a luthier friend of mine to have a look at the page and offer an opinion. I'm personally not convinced - and it's of no use to electro-acoustic guitars with the volume wheel just inside the soundhole rim.

However, if anyone has a practical test and reports improvement, I'd be very interested.