The i5 2500k is about as easy to overclock as any and runs pretty cool at overclocked speeds. I tend to hit thermal walls sooner with Phenom chips, although they do have a slight amount more flexibility when it comes to memory voltages.
i5 + 1.5 volt memory will pretty much eat anything for breakfast up to the thousand dollar chip mark.
The comment about the i5 "destroying" the Phenom is somewhat dramatic, but the truth is somewhere in the middle. In my personal experience the Phenom II chips are more geared to compete with the older Core 2 series and do well in that regard. Generally they need 400 - 700 MHZ more to achieve parity with SB. A 3.0 GHZ i5 for example, might have the same performance as a 3.5 GHZ Phenom II.
The price difference is -not- as great as some people might claim, though. On Newegg, comparing an 1100T and the i5 2500K, factoring in motherboards with similar features... roughly $45 total gap for more overclocking headroom and better performance. YMMV depending on part selection, but the platform cost differences often get grossly overstated.