If we can take anything away from the 9xx series it is this.
1. The 1080 will only be 15 - 18% faster than the 1070 for twice the price.
2. The 1080 will be another 'red headed step child" just like the 980 did once the Ti comes out.
3. Once prices stabilize, one might expect, two 1070s in SLI will have a lower cost per fps than the 1080 by a significant margin. However, this may not happen as CPU, RAM and who knows what may become limiting factors. In addition, where games that didnt support SLI were rare, of late, more gaming titles have FPS limits and / or console ports that the dev was happy to take his console cash and go home.
4. Still getting one 1070 for < $380 once prices stabilize makes a lot more sense than a $650...