Need or Want to Upgrade?
So, is there any need for those still running the Core i7-5820K to upgrade? In short, no, and as mentioned in the intro, we largely knew as much before even running the first benchmark. For gaming, the gains simply aren't there and we'll take a closer look at that in a moment.
Moving away from gaming for a second to clear up one exception: if you've overclocked your 5820K to the max it's still not getting the job done quickly enough, then yes, an upgrade can help reduce workload times.
For productivity workloads, the overclocked 8700K was 20-30% faster, so it's your call on whether or not it seems worth to buy a new CPU, motherboard and potentially higher-clocked DDR4 memory.
It's worth noting that the Ryzen 7 1700 is $100 cheaper than the 8700K and for most productivity workloads it is as fast or faster with both CPUs overclocked. There certainly are applications that still favor Intel CPUs, so make sure you research how these CPUs compare in the applications you'll be using.
If gaming is the focus and application performance is more of a secondary demand, then right now Intel is the best option, at least for flat out performance with the latest and greatest GPUs.
Here's a look at the average results seen across the six games tested. Here the 5820K was just 10% slower and that's seen when using an extreme GPU on mild quality settings. That's certainly not a big margin and I can't imagine anyone would ditch their 5820K for that kind of gain.
Under more realistic conditions we see just a 6% decline in performance when using the 5820K as opposed to the 8700K and you can expect that margin to at least halve again when going to 4K, if not evaporate entirely.
So it's not worth upgrading from the 5820K to anything newer for gaming, we've established that much. Productivity is a little more tricky but that really comes down to how much money you're willing to invest to reduce your downtime by about 20-30% and even then, you're better off looking at CPUs with eight or more cores for a truly worthwhile upgrade.
It's a complicated buying decision and while some people like to try and simplify it with their own bias by saying either Intel or AMD is better, the truth is that it's far more complex than that.
Second-hand shoppers... assuming a realistic $250-$300 price for the Core i7-5820K and X99 combo, that's at least 40% cheaper than the $500 that an 8700K and basic Z370 board will set you back -- a big difference, especially since the performance difference for gamers is negligible. Of course, second-hand hardware comes with obvious drawbacks such as a lack of warranty for example, but 40% off may be worth the risk to some.
The other alternative would be a Ryzen 7 1700, which when coupled with a decent B350 board can support a 4GHz overclock in many cases and the combo only costs $360 or so. In other words, you'd be saving $100 at most by going the second-hand route, and realistically probably less than that. While gaming performance will be as good or better with the 5820K, many productivity workloads will be boosted with the Ryzen 7 CPU.
You also get a warranty with the brand new parts, so personally I'd shy away from the second-hand market in this situation. Additionally there's now also the security issues that come with older Intel hardware and we're not 100% sure when CPUs such as the 5820K will receive the required BIOS update to help mitigate those concerns. That update will come with at least a 3-5% performance hit in games so be aware of that. I believe MSI just rolled out updates for X99 boards but again, note that all of this testing was done pre-patch.
In short, if you're a gamer and you already have a 5820K, keep it for now. If you're looking to buy second-hand, it seems better to aim for something cheaper like a Haswell Core i7-4770K or 4790K. Unless you're getting the 5820K with a motherboard for less than $250, we wouldn't bother. AMD's recent Ryzen price cuts really make products like the R5 1600 and R7 1700 mighty tempting but even then it might pay to hold out for a few more months and see what the Ryzen refresh brings and what that does to prices.