The difference is not as big as I expected, but is certainly noticeable and welcome. With the valves closed the difference is minor, so you can still cruise down the motorway in relative peace. But with them open, it's deeper, louder and more satisfying. Overall my instant reaction is that it's a worthwhile mod for little cost.
That said, I can see why some fit sports cars as well to free up yet more noise. And maybe that's something I'll think about for the future. But for now, for me, it's loud enough and fulfills the brief.