You could always wait for the next best thing, but how much time do you want to spend waiting? The way I see it is you get something great now and it gives you a 2-3 years of great gaming performance... Great! Then you can consider the next thing. Plus, with a SLI-capable motherboard, you have great expansion capabilities. Maybe pick up another 680 some time later, if you feel it necessary.
I say go for it and don't look back. Who cares if something better comes along in 4-6 months? For now, you'll still have the best performing GPU the consumer market offers until then, and it will continue to be a capable card for the next couple of years compared to any others. And like I said, if you get an SLI motherboard, you could always add another 680 at some point. I'd recommend a board with PCIe 3.0 if you're building new.
Also, if you get a 680 (or two), the next time you have budget to upgrade, you'll have a decent card(s) with some resale value to help pay for it.