The new Intel i7 4790K isn't anything amazing when compared to the i7 4770K that it's replacing. That's not a bad thing, though. For starters the i7 4770K is and was a fantastic processor. It offered great out of the box performance and overclocking is extremely easy. With little effort you could increase the clock speed easily in excess of 500MHz with little adjustment to the voltage and little added heat.
The other big thing is the simple fact that the new Intel i7 4790K comes in at the same price point as the previous generation processor. This just straight out replaces it. You'd be kind of crazy to move from a 4770K to a 4790K. The upgrade just isn't worth it and there's no way you would get the money back on selling your current 4770K to justify the upgrade.
At the same time you'd be crazy to not buy the i7 4790K if you were looking at buying the 4770K previously. If you're going to grab a Z97 motherboard and you want a K series based i7 processor; this is going to be the one to buy. If you're not going to overclock, though, most people would say go down the path of the non K based i7 4790.
At around $25 cheaper we'd say make the move to the K one even if you're not overclocking assuming your budget can afford it. First it's going to give you a better resale rate down the road. Secondly if you don't know how to overclock but someone you know does; they could easily just come over and give you an added 400 – 500 MHz in a blink of an eye. Of course there's also the fact that it's got a base clock of 4GHz which is also nice.
In the end the Intel i7 4790K slightly improves on what is already a fantastic processor. There's no reason to not buy this if you're looking to spend over $300 on a processor. Combined with the slew of new Z97 motherboards that have hit the market; this paired with a nice high-end single GPU Video Card from AMD or NVIDIA is going to make for a truly wonderful gaming system.