I'm running a GTX980Ti. At 1080p60 it has handled everything I've needed it to with minimal quality tweaking. I tried multi-monitor gaming (I run 3x1080p & a 1280x1024) but found that while it ran smoothly enough, it was more hassle than it was worth.
In general I assume that it's not worth upgrading more often than every third generation unless something truly revolutionary comes along. The only revolution I've seen in recent years has been nVidia's pricing :-(
I'm going to wait until I've seen 3rd party benchmarks for both the 30-series and whatever AMD comes out with before I consider upgrading.
TBH I'd question the wisdom of anyone who buys one of these cards in a hurry.
Or I might hold off another year and put the last of this year's money towards a 1440p monitor instead.