Back in July, I was able to get the 4790k at MC for $280. And then my mobo was $150 rather than $185. And then my ram was $150 rather than $175. So, that's more like a $250 difference, which is about the price of a 290 which would let me play many games at 1440p. The 4790k definitely has a place for certain enthusiasts, as you could even go with 8gb of ram and a cheaper mobo, saving an additional $125 or so. And single-threaded performance out of the box (or if not overclocking) is faster than a 5820k.
If I were buying today rather than 5 months ago, perhaps I'd go with the 5820k, but it's not as obvious a decision as some of you are describing. 6-8 cores will eventually be commonplace, but I think it's going to be a few years before it becomes 'necessary' for gaming. Heck, we're really just now getting to the point where dual core CPUs aren't capable options for gaming boxes. And almost always, a faster graphics card will be a better way to invest dollars for any gaming machine, so getting a 'good enough/bang-for-buck CPU' should be the priority. Honestly, the 4690/k is probably the best price/performance ratio, but I decided to splurge on the HT and higher clock speed of the 4790k so I don't have to overclock.
As for when we'll need 6+ cores, my guess is that with 6-core variants of CPUs not being a part of the mainstream socket until at least after Skylake, I think we have a few years where a 4790k will perform at least as well as a 5820k in gaming performance. When 6-core variants start being available on the mainstream socket, that's when we'll truly see a shift. At least IMO. And by that point, I'll likely be ready for a platform upgrade.
One other bonus, the 4790k has an iGPU, which while not useful for any real gaming, does offer a nice emergency backup if your old GPU dies. I'm actually using it right now as I wait for the Sapphire Tri-X to drop down to an even $250.