I don't know if it's putting the person down or letting the person know what he's in for if they have it.
Personally, whether I just bought something or was thinking of buying something, if I asked for an opinion, I'd expect just that, an opinion. Most people that have some decent tuning skills always dissuade people from those kinds of pipes. They are built for show, not performance. From what I've read, the people that have had them, the pipe causes an erratic tune that can't be found, power is degraded and temps are all over the place, but generally hot.
If I had just bought the pipe and someone told me all of that, there wouldn't be much I could do, since I bought it. But, if I hadn't used it yet, it would be enough for me to return it and get my $ back. If it's used, you won't be getting your money back as easily.
EDIT: guess I don't type fast enough...