I'm enough of a doubting Thomas that people often mistake my place of origin as Missouri, particularly when it comes to gun "gadgets." That said, I approached Hans Vang's system with a great degree of trepidation. I was amazed to find that I was wowed by it!
The groups we've gotten out of 18" 870 barrels, nominally bored "Cylinder" from the factory, have been astounding. OO Buck well within 4" at fifty feet is not uncommon.
One potential downside I can see to this lack of typical shot spread is an increased demand for accuracy on the part of the shooter. "Downside" is probably an improper term here; every shooter should strive to be as accurate as possible. Nevertheless, many people, quite erroneously, think they can rely on the spread of a shot pattern to compensate for a poorly-aimed shot. Needless to say, the tighter the pattern, the greater the need for accurate shot placement. In working with students in our defensive shotgun classes, I've noticed that those shooting Vang-Comped guns tend to initially miss a bit more, then begin to use their sights and take better control of their hits. They normally exit as the better marksmen of the classes.
I normally despise compensators of any type on defensive firearms. For some reason, however, the Vang Comp does not seem afflicted with the problems associated with most comps, namely fingers of light probing the retina at night. Even when fired in a controlled situation in a virtually lightless indoor range, the comp flash was not much more than usually realized from the muzzle.
What it does do, however, is it's assigned job very well. The Vang is one of the most effective compensator systems I've ever fired on a long gun -- double-tap times of well under a second were recorded repeatedly with a Vang-equipped, action-slicked 870.
As I write this from my home office, a glance to my right discloses the 870 we did our original Vang testing on. It's in an Insta-Guard rack on the wall, replete with S.G.T. ghost ring sights and one of Vang's barrels. I thought enough of it to bring it home -- about four years ago.