Depends what you are looking to gain from it, what games you play also. Personally, I try to wait every 2 series before buying, that way I can justify the cost a bit more. If you spend about $300-$400 each time, wouldn't you rather a bigger gain? That's just my opinion, and I'm sure you'll get plenty differing ones.
Asus G20CB - Core i7 6700, Nvidia GTX980, 16GB Ram, 256GB SK Hynix SSD, 1TB Samsung SSD.
I was thinking about this too. Imho it's not worth it unless getting a 1080. Demos I've seen playing what I play, battlefield 4, my 970 with a core over clock of +150 and memory +400 I am hitting ~110-144fps on 1080p ultra, while a 1070 seems to maybe outperform just a bit more. To me I'd love a water cooled 1080 but the prices are insane so I'll wait to for 1170 or 1180. All depends on what you play and your monitor of course.