I recently installed a i7-4790K.
Since the retail prices are the same, there is no downside to the newer version.
But... some of the old units will get sold at a discount.
My impression from a limited time with it and a previous 4670K is good.
If you load the optimized defaults and run at 4.0 to 4.4 it runs coo and well.
When overclocking, you will run into the same limits at 1.25 volts and higher than previous 4770K did.
Namely heat rises rapidly past that.
My take is that for a new build it is worth it, but do not expect to see 5.0 overclock like early speculation.