There are two ways to evaluate the Core i7-8086K. The first is as a commemorative curio. Intel fans who want to celebrate 40 years of x86 should probably leave theirs sealed in the box for future generations to pick over when silicon becomes a curiosity. Intel is only making so many of these, and sealed copies will likely become more and more unusual with time.
As an actual processor, the i7-8086K isn't worth the $75 upcharge over the i7-8700K at stock speeds. Outside of its rarely-seen 5-GHz top Turbo bin, the i7-8086K performs the same as an i7-8700K the vast majority of the time. That's because the rest of its Turbo Boost 2.0 table is identical to the i7-8700K's. There's only so much a chip can do within the same thermal budget. It would have been nice to see Intel really take the leash off this thing and push TDPs or implement something like its Thermal Velocity Boost feature on this chip to truly make it something special for those who don't want to overclock.
The story changes a little—and I do mean a little—when we take advantage of the i7-8086K's unlocked multipliers. It's tricky to recommend a processor on the basis of its overclocking prowess alone, because no two chips will overclock alike. That said, our retail i7-8086K made it to 5.1 GHz on all cores without any AVX offset and nothing more than the usual thermal challenges of modern Intel CPUs. No i7-8700K in our labs can run at speeds higher than 5 GHz for non-AVX workloads, and they require -2 AVX offsets to remain stable.
For all that, the i7-8086K's slightly higher overclock didn't translate into many practical performance benefits in our tests versus a run-of-the-mill 8700K at 5 GHz. Still want to pay that $75 extra?
That behavior does suggest Intel is putting its best Coffee Lake silicon of late under i7-8086K heat spreaders, so overclockers who are looking to get the very best performance out of their chips might not mind the upcharge. Third-party retailers like Silicon Lottery offer binned, delidded i7-8700Ks for more than an i7-8086K goes for at e-tail. If the i7-8086K proves its mettle as more enthusiasts get them in their hands, the price of this CPU could be plenty reasonable for those after the very best Coffee Lake dies.
Ultimately, the i7-8086K is more of interest for its history-marking and history-making than it is as a practical processor for the enthusiast. If you want one and don't mind the fact that you're paying for what is essentially the pleasure of special packaging, well, fair enough. Everybody else should just buy an i7-8700K or a Ryzen 7 2700X, depending on whether high-refresh-rate gaming or multithreaded grunt is what's called for.