Summary: i paid the extra 20.00 bucks assuming i could squeeze out the extra performance. while overclocking on my asus z87 motherboard using a thermaltake nic c4 cpu cooler. i found that I COULD NOT get the cores over 4.2. i was content with this speed. the cores must have a bad clock. i have moved voltage to the appropriate leves. never exceeded 2.25v while testing. even though i was able to boot windows at this multiplier, it would crash when stress testing.
Summary: The 4670k is the most powerful i5 as of the time of writing. It is more than enough for gaming,and suitable for most other tasks also. A professional might consider the 4770k instead for its hyperthreading capabilities. The only thing is that the haswell has poor thermals. Not very good for overclocking and enthusiasts. There is very little reason, if any, to upgrade from a 3570k if you have one.
Summary: This is a very nice CPU for the price and the feature set is appropriate for most users. The CPU stays very cool with my NHU12S and seems to always run at the boost frequency of 3.8GHz. So far it has been very stable for me at stock speeds and it's definitely a speed upgrade over my Phenom II X940BE.