If you look at the data these guys took:
http://www.rouesartisanales.com/ the HED3 is among the least stiff and most aero wheels around.
That said, they're nice wheels, and I raced a pair of really old ones (they were actually specialized tri-spokes) for a couple years at a pretty high level. The only time I ever had an issue was heavy side-loading during bad madison exchanges on a concrete track causing chattering. I know someone else who had similar problems with them, but only on wood tracks... They were great for pretty much everything else, and it didn't really stop me from riding them in madisons, either. They took quite a beating, surviving more that a few crashes, but I did finally crack a spoke in one a few years ago. I haven't checked prices on them lately-- I got the pair of tubular trispokes for ~$500 or something like that several years ago, so $680 for one seems like a lot.