The D600 is a very good arm, but don't confuse weight at minimum extension with weight at full extension. The Avenger D600 boom arm will support ~15.5 lbs. fully extended (83"), not 60 lbs. The 60 lb. rating is at minimum extension (46"). If you've already got a heavy-duty stand to connect it to, I agree that it's a better way to go than the Photoflex arm.
Regarding longevity, I've hung a head and 3'x4' softbox off of a 3397 for over 10 years and never once had a problem. I like it because the arm retracts into the body of the stand, which makes it very portable. You don't have to dismantle the thing to transport or stow it. It also doubles as a 13.5' light stand. Bottom line for me is that it's very versatile.