i7-2600k's money worth comes from Hyper Threading which adds 4 logical threads on top of the original 4 cores. This helps the CPU work to it's fullest when using an application capable of >4 threads or multi-tasking.
If you can utilize 8 threads at once the i7-2600k could very well be worth the cash.
Don't expect a 45-50% increase in performance like the increase in price, but the performance is definitely good if you can harness it. I would expect 15-30% performance.