You'd Be Surprised
First thing's first - do the Killer cards really work?
You might be thinking the answer is a big fat no. Certainly, the Killer has been subject to a great deal of skepticism. This is no surprise, considering that what it says it accomplishes does not seem like something a network card should be able to do. After all, your latency depends on your ISP and various other external factors, not what is in your PC. At least, that's the common wisdom.
However, your network adapter can actually have an effect on not only lag, but also on your maximum upload and download speeds, the way a game feels during heavy lag, and the way a game responds to being played in conjunction with other tasks which also make use of the network adapter. Slowdowns that are caused by your network adapter are what Killer calls "client-side lag," and Killer's goal is a network card to rid your computer of it, resulting in a smoother online experience.
There are several ways Killer kills client-side lag. The card has custom hardware that takes on the load of processing network data that might normally be sent to the processor, thereby lowering processor utilization. The purpose of this is to try and enhance average frame-rates by keeping the processor's attention on running the game instead of the network adapter. The Killer also has a custom packet prioritization scheme which works to make sure that data relating to any game you might be playing is given first dibs, be it incoming or outgoing.
The result of this networking wizardry is difficult to quantify. Many reviewers who have taken the Killer to task have noted that getting accurate pings is extremely difficult. Pings will differ depending on the server a player connects to, the number of other players connected to that server, network traffic at the time, and etc. In addition, the only ping information available is through the game itself - provided that the game even gives real-time ping information, and most don't. These problems underline problems with the Killer itself - it is client-side only, and so it cannot control the many factors outside your computer which may cause lag.
However, while objective testing of the Killer NIC has proved difficult, most reviewers have stated that the Killer does provide a better experience. The difference is not night-and-day, but the Killer's custom hardware seems to do its job. Stuttering, lag bursts, and other such phenomena feel much less common and severe when using a Killer.