Okay, so updating the first post...
Yes, overclocking your ES skylake is 100% possible. You can do it even without the non-k bios (at least, with the Asus z170-k I was using!).
Keep in mind, though, depending on the model you use (the QHQG is the one I'm using) has a low multiplier that would require a BLCK over 170 to reach stock i7-6700 speeds.
Most motherboards using the standard clockgen cannot handle this. Your board will have severe stability issues, and will crash a lot if you can get your board to start at all. More voltage to the core, or to the PLL can fix this to an extent but you WILL reach a wall where you'll get such huge diminishing gains it's impossible to get any farther. You'll need to get a Z170 motherboard with a dedicated clockgen.
So to recap.
OCing is possible. You don't lose AVX instructions, temps read out ok and I'm pretty sure the integrated GPU still operates fine. Turbo boost also works, as well as speedstep. So, basically every issue with normal non-k OCing you shouldn't have, which is obviously a huge bonus.
Now, I'm also going to say this was MY experience with this. I do not know if this will apply to all ES CPU's, or to all motherboards, so take these results with a grain of salt. If you can get it to work, you can get a really nice CPU for considerably cheaper than normal price.