Well as the owner of a 970SLI rig for the last two years I can't praise enough my experience with a dual card setup.
All the games I play work flawlessly with it (BF4, GTV V, W3,Project Cars) enabling all of them to run at a solid 60fps with maxed out settings at resolutions up to 1440p, and they are all playable to a locked 30fps at 4K.
Two 970's are still good enough to this day, so much so that I can easily skip this first generation of Pascal cards, as the only card with any sort of performance gain over 970SLI is the 1080, which is only about 25-30% faster, so not really worth it IMO.
If anyone is unsure of 1070SLI, I'd say go for it, they will easily give you 2-3years worth of 60fps, maxed out gameplay.