Depends on how you define good. I got 386 cpus from 1991 that still run, but is it really good? I still got lab machine running on Core 2 Quads Q9550 from 2009, is that good? The CPUs usually won't cook themselves unless you've been overclocking them to death.
Yep, I would think so. Considering the speed at which the CPU market has moved forward in the past, I would say a 6 core CPU is a safe bet for, oh, around 5 years, maybe even more. While 8 core CPU's are also mainstream now, but Intel hasn't jumped on them yet, and developers like to optimize for all CPU's available, so they'll probably to stick to optimizing for 4-6 cores for now, so that 8600K will be good for quite some time.