The GTX 960 is sold as a replacement for the GTX 760. It is faster, uses less power and is cheaper than the GTX 760 was when it was released.
The GTX 970 was a replacement for the GTX 770 and ticked all the same boxes as above.
The fact that the GTX 960 is so close to the performance of the older GTX 770 or R9 280 is remarkable given it's price and power usage.
For 1920x1080 resolution with medium to high settings, the GTX 960 looks to be a very good card.
If you want higher detail settings, higher resolution, multiple monitors or performance over 60 FPS, you need to be looking at a more expensive card.