I have a Core 2 (E4500) from 2006 and a Sandy Bridge quad i5. I upgraded for gaming, but the difference in day to day use was much more than I expected. They both have conventional hard drives, so the difference is not due to an SSD. I did not expect much improvement in day to day use, but in my experience the difference is remarkable, and would definitely be worth the cost, even if I did not game at all. Now if you are coming from a high end Core 2 duo, or have a good overclock, it might be different. But going from a stock E4500 to a stock i5 2320 was a huge upgrade in day to day use.
I do have a feeling going from my i5 to haswell or skylake might not be as noticeable in normal usage, but stock Core 2 duos are getting *really* long in the tooth.