I've got it on my 4.4, and I love it.

Here are my thoughts, and an excerpt from that post:

The Remus has subtle and understated but noticeably deeper tone compared to stock -- louder, but not intrusively loud. I previously described it as a somewhat throatier sound which is louder and definitely more aggressive when goosing it while driving, but not appreciably louder while at idle. The improved sound is quite noticeable when I punch it into the higher RPM range while driving, as opposed to revving it while parked in neutral. Definitely a subtle, yet quite noticeable improvement that, like a fine wine, only gets better with time.

Both Eisenmann Race and Dinan are appreciably louder than the Remus, and both have their ardent supporters.

Overall, I really like the subtle, deeper sound of the Remus, and the dual 84mm tips are quite sharp -- really nice looking tips. It’s a quality product at a lower price point than either Eisenmann or Dinan. Also, like Lindsay Lohan’s enhancements, Remus is a bolt-on.

Just to give you an idea for comparison purposes, check out #3 in the “Breaks, Exhaust & Suspension” section of X5World for a comparison of Art’s 4.6is Dinan v. SuperGreg’s 3.0i Remus. That's a classic. Bear in mind that the Remus on a V8 will sound louder than on an I6, and that Art’s 4.6is had the full treatment of Dinan upgrade goodies. I think that x5Monkey had previously posted a clip of his Remus on his 4.4, but I haven’t been able to locate it.
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I don't have a clip of the Remus on my 4.4, but here's a link with a clip of a Remus drive-by on SuperGreg's 3.0, and here's another Remus clip (think it's also a 3.0, not sure).

It's fairly quiet at idle and low RPM's. I can hear it inside the cabin if I goose it, if I'm driving through a parking garage, or sitting at the driveup window of Dunkin Donuts -- especially with the window(s) open.