I'm using some for the first time recently, it's good stuff. Low voltage, low (enough) profile, and acceptable pricing. Their online support and openess through their forums was very encouraging too.
I had been interested in trying them out before but I just don't build/upgrade machines often enough these days, so finally got some for my sandybridge build a couple months ago.
It's not something I'd be too crazy about though, I mean, I wouldn't be prepared to take the pepsi challenge against other RAM or even claim I have the bestest, but still a sufficiently happy/satisfied customer and I'd have no problem plunking down for some more in a future build if things stay the same.
Previously I tended to gravitate towards Corsair Dominators (pricey for what you get though IMO) or XMS3's.