Well, it's cheaper than a HD 7970 new...
So you aren't impressed with the GTX 680, but you have a 3rd on the way...hmmm
Being the owner of two cards each, I will say that I am a little more happy with the HD 7970's. When all cards are overclocked to their potential, the HD 7970's comes out on top in performance, so far from my testing.
However, for gaming, I do enjoy PhysX with PhysX games, and Keplar also will allow TXAA implementation, which looks pretty cool.
For an all-around card, the HD 7970's are better with their overclocking and compute prowess, but for gaming, the GTX 680 does what it needs to. Not to mention the GTX 680's stay cooler and provide less power consumption than HD 7970's, which get really hot, even under watercooling.