I just purchased a Benz silent rider exhaust for my 450 Grizzly:
http://www.atvsilencer.com/When you are sitting on the atv riding it, it sounds about 10% quieter (barely noticeable). However, to an observer on the ground the noise is at least 50% reduced - very noticeable improvement.
Power off idle to mid range is unaffected (possibly improved). Top end before the exhaust was 53, now it will do 49mph (not a big deal, don't ever ride that fast...)
Is it worth it? I don't know yet. I'm trying to give it a try and see, but mostly what I've seen is that instead of seeing deer trotting away from the ATV at the edges of my vision, I'm seemingly surprising the deer by getting closer to them, which in turn causes them to bolt in terror from close in.
I'm leaning towards taking it back off...
David