Nowadays Overclocking is just another premium market segment being milked by Intel with K/Z series chips.
Every single Intel chipset is 'in theory' capable of overclocking (multiplier change, Turbo). Its all blocked on the Bios/OEM agreement[1]/microcode(!)[2] level. Sometimes OEMs grow bold and ignore it (Asrock pioneering H81 overclocking), sometimes they get spanked by Intel and quickly release 'updated' bios removing OC.
OC in itself works great. You an buy $72 3.2GHz G3258 haswell, OC to 4.3GHz (stock cooler, or ~4.5Ghz on custom) and enjoy >30% speed improvement. Especially great for single threaded games(written in python!) like World of Tanks where your oced $72 cpu performs like $300 4790K.
[1] http://www.digitaltrends.com/computing/asrock-killed-overclo...
[1] https://www.reddit.com/r/overclocking/comments/3ji7im/nonz_o...
[2] http://www.sevenforums.com/windows-updates-activation/373250...
These firmwares disable power management completely, so that the microcode cannot even read the BCLK frequency and detect overclocking. As a side effect, you cannot use integrated graphics, and AVX instructions become slow — even slightly slower than at stock frequency (need power management to ramp up the AVX parts of the chip to full speed). But if you're not encoding video all day, you can save some money by buying a 6400 instead of a 6600K.