They sound GREAT!!!
Not quite as deep as Termis on a Duc, and MUCH angrier sounding on top. I'm VERY, VERY happy with them.
I could never call them "quiet," though they're not anywhere near as obnoxious as most of the Hardleys around here. They've held up very well over the last 2 years, with only a few stains to show for it. They're due for a repack, though: A couple weeks ago I went by a parked SUV at 250MPH, and set off it's alarm.
Click on the link to my photo album: There's a dyno chart in there showing stock, pipe/chip, and pipe/chip/airbox curves. All on the same Dynojet 150, by the same operator. The all stock curve was taken about a year before the others, but the pipe/chip and pipe/chip/airkit runs were done within an hour of each other. We actually did the mod with the bike still sitting on the dyno, so it's a true before/after deal.
Oh yeah - the guys at Evoluzione Motorsports did a big pipe comparison, and found that the stock headers on the RSV are actually pretty good: Spending the extra cash on a full system doesn't gain you as much as you might think, though should still be necessary if you plan on doing head work.
Hope that helps!