My brother has one.
I don't notice squat of a difference between his and my onboard realtek, or intel in gaming.
I do notice a difference when transferring files from one to another tho.
The intel nic hands down is a tad bit faster.
The biggest peeve I have with the killer nic is when setting up windows.
Intel nic's and realtek nics are detected mostly at install.
There is no additional drivers I need to install, so once I get OS installed, I can goto motherboard's web page and download the drivers.
However with my brothers system, since it also has no optical drive, I would need to either transfer over the driver onto a USB flash, or put in a usb optical and do the drivers that way.
Also at windows bootup, it seems to take a couple of seconds for the nic to come live, while as on my intel, the moment I see desktop, its already connected.
Lastly, I prefer dual intel nics over one intel and one killer.
The reason being under most cases one wont have any issues with cpu overhead.
Unless ur using a really underpowered PC, which then a system which uses a killer nic would be out of your price niche, I don't think anyone will have an issue with cpu overhead.
But this is just my 2 cents.