I've done this upgrade. Up front, there are a small number of games that do benefit from this, such as gta v or cryengine stuff, but most will get no change whatsoever, I can guarantee you that. I can't speak for the future though.
This is the kind of upgrade you want rather than need. An overclocked Sandybridge WILL see you through another year, if you can live with very slightly less gloss in a small number of titles.
Ultimately, if you want a big boost in games, your Sandybridge will still power a nice GPU upgrade. The reason I didn't just do that is because I feel its too late in the cycle to drop £500 on another 28nm gpu; i decided to update the rest of my platform instead. Thats my own perogative and i will be upgrading to a pascal/polaris later in the year

I also wanted to replace my aging chipset platform, and for that alone, z170 is indeed a big step up.
So in short, you don't need skylake, or even x99. But this is an enthusiasts forum, not a savings bank. We're going to see plenty of people here making the leap, and I say HELL YEA!