The Intel i7-6700K is a high end 14nm Skylake based 4 GHz quad-core CPU. It is in Intel's K line and so is unlocked allowing for easy overclocking. It fits with the LGA 1151 socket for motherboards that feature it.
Pro Can allow for extreme gaming solutions
The chip itself allows for 16 PCIe 3.0 lanes (which was not available in such high numbers in the previous generation chips). This translates into how many PCIe devices can be used at the same time and how fast they can communicate, which makes for a scenario where multiple PCIe GPUs can be used at their top transfer speed of 16x for one and 8x for two.
Pro Able to be paired with DDR4 RAM
Being a Skylake 6th generation CPU this chip is able to be used with DDR4 RAM as this is what the new LGA 1151 based motherboards are able to use. DDR4 is available in faster speeds over DDR3 RAM (what was used with previous generation CPUs) meaning an overall speed bump in RAM when using this newer technological chip.
Con Adds another pin to the connection
This processor uses the LGA 1151 CPU socket. For those upgrading from an older generation chip this means the will need to purchase a new motherboard and possibly RAM in order to use this chip. This can be expensive to replace so many components all for a single new pin in the chips connection. While the performance will be better than the last generation, it may not be worth the price of all new gear to see this performance.
Con Does not come with a heatsink
No longer does Intel include a heatsink with their new CPUs, so the user will have to purchase their own. While they are not too expensive, that basically means out of the box this can not be used unless a purchase was made for a heatsink along with this item.