I have a 1440/144hz G-Sync monitor, and I love it, and I've had it paired with both a 970 and now a 1080. When I got the monitor I upgraded from a 1080p monitor and I was worried my 970 would struggle at the lower FPS. But g-sync made all the difference. Instead of getting 70+ fps I was now getting ~50 but it felt just as smooth if not more than before.
Now that I have a gtx 1080, my frames are even more smooth as they often reach a constant 144hz. I think the true value of a g-sync monitor is that it might let you stick with the same GPU for a little longer, as you won't notice when its performance starts to lag behind in newer games due to g-sync keeping things smooth.
So with that said, if you don't play extremely demanding games and are going to be able to keep the games locked at your monitors refresh rate then there really isn't a whole lot of need other than "future proofing" so to speak. If you're sticking at 1080p and only play CS:GO and e-sports games, it might not be necessary as your 1070 is going to pump high fps in most games, especially if you're willing to tweak settings. But if you're going 1440p and/or you want to be able to play AAA games for several years without upgrading GPU (or if you're very loyal to Nvidia cards) then yes go for G-sync.