What do you now own?
What are you trying to do?
What is your budget?
Worth is something only YOU can determine.
I5-6500 runs at 3.2 with a 3,6 turbo.
I5-6600K runs at 3.5 stock with a 3.8 turbo. More like 4.5 if you have a overclockable z motherboard.
How important is cpu power to you?
Is the 10% price difference, and the 10% performance difference worth it to you?
I