When we tested the Core i3-7100, we concluded that the new Kaby Lake architecture brings no significant performance gain over the former one, Skylake. The only real performance improvement, according to Intel, is on 4K video playback.
However, if you must choose between the Core i7-6700K and the Core i7-7700K, the last one is a better deal, because it has a higher clock and it costs the same as the older CPU.
Comparing the Core i7-7700K to the less expensive models, it is clear that it is a lot faster on the applications that use all the processor threads. On applications that use less than four cores, the performance difference is small.
On games, it was clear that the Core i7-7700K was faster than its cheaper brothers on some titles, while on other games there were no difference, even with a high-end video card. So, if you use a lower-end video card, probably only a few games will take advantage of a faster (and more expensive) CPU like the Core i7-7700K.
So, we concluded that the Core i7-7700K is an excellent CPU, very fast on any situation. It is recommended mostly if you need high performance on parallel tasks (like 3D render and video editing) or if you are building a gaming PC with a high-end video card(s).
On the other hand, if you have a limited budget for a gaming PC, the golden rule is the same as always: it is better to buy a mainstream CPU and save for a better video card, than buying a high-end processor and a cheaper video card.