Check CanonWatch.com and CanonRumors.com; they routinely post great deals for this lens. Part of the reason it's widely available for sub-$700 prices, new, is that it's a kit lens included with a couple of bodies--and retailers who split the body and the lens up are willing to sell the lens for that price. I suppose those retailers are getting for example a 6D + 24-105 kit for $1900-ish, splitting them up, and reselling the body for $1600 and the lens for $650, and still turning a profit.

Now, there could be drawbacks--maybe, possibly the warranty? Still, we're talking a 1 year warranty; after that, it's pay the service center fee regardless, with the possibility of slight discounts as a CPS member (Canon cheesed down even the Platinum service discount in September, though).