I've looked vehemently into the 680's benchmarks and efficiency.
In terms of efficiency, it's the easily best graphics card ever made.
In terms of performance, it's arguably the best single-GPU graphics card ever made. With day-one drivers. A few updates from now, this is probably subject to change.
There's a rumor of an upsized version of the 680 based on a different core (called "Big Kepler") currently floating around in the development stage, but it may just be a pipe dream. I plan on replacing my two GTX 260's with a single 680. The performance differential is astounding. tl;dr I love the card.