I'm still rocking an i5-2500 from the same generation.
It is still completely fine for everything I ask of it even against the much newer machine at work, with the upgrade to an SSD a while back it basically felt like a new machine.
I upgraded from a i5-2400 to a second-hand i7-3770K around a year ago given they're socket compatible: the difference between them is larger than the difference between the i7-3770K and the i7-6700K, from what I saw looking at benchmarks when I did this. There's certainly workloads where there's a noticeable different in performance.
I have an i5-3570K in the work desktop, I really don't notice that much difference though the i7-3770K was/is a beast in comparison, for my workloads I just don't see much benefit in the i7's, I'll likely get another i5, all the ones I've had have been excellent on the $/perf scale going back 5 years or so.
