Favorite thing about the 900 series in general? Power/Performance.
Lets contextualize how amazing this series is. A GTX 660 has a 140W TDP. 660s in dual SLI, from what I've seen in researched benchmarks, perform at about the level of a 780, which is slightly above (or in these cards' cases, even with) the GTX 970. We're talking a low single digit % difference. A single GTX 970 has a higher TDP by only 5W, meanwhile 660s in dual SLI have a 280W TDP.
In two generations, NVIDIA was able to increase the amount of performance you get out of around 140W of power by approximately 90%. Not only does this mean the cards use significantly less power for the performance you're getting, but they can run quieter, cooler, last longer, and base/OC clock speeds on the higher end cards are able to be as high, or even higher, than not only previous high-end NVIDIA cards, but any graphics card before it.