I agree, the older 721's seemed to last a lot longer but the last 2 I bought which were silver died a quick death.I'm running Mavic 721s on both my bikes front and rear. I'm just bummed that they stopped making the CD (hard-ano) version. Sounds stupid but I swear they were stronger.
Oh and Alex Supra Ds are absolutely worth checking out - at 620g they're heavier than the 721s but not by a whole lot and they don't flat spot as easily.
Only weakness with the 721s is the braking edge which seems prone to folding.
The Alex supra D is a much better rim than the current 721 imo, claimed weight is lighter too. A lot of guys are starting to switch to these down here now.