I've always been tempted to get myself a Killer NIC, simply because I enjoy keeping my machine on the cutting edge of technology...but honestly I cannot justify this card. I see three basic scenarios:
1) Slow/aging computer. Would this PC would benefit from a Killer NIC reducing CPU load? No. For a fraction more than $200+ this card will set you back, you can buy a budget mainboard, a dual core CPU and 2GB ram. Sure, it's not gonna be amazing...but the performance increase will be significantly more than a few percent.
2) Mid-spec "average" computer. Nobody with an average specced computer (read, not the sort of person that sees their computer as something to spend all their spare income on) is going to care enough about a couple of percent increase in performance. They'd be better getting themselves a new graphics card and some more ram.
3) High-spec "enthusiast" computer. If you have a top-spec machine because you know your stuff, then you'd know that getting a cheaper Intel Pro/1000MT card is the better option. If you have a top-spec machine because you have more money than sense, you'll get the Killer NIC, simply because you want the "best available".
With that said, it would have been nice to have seen a comparison to a decent TOE enabled Intel NIC, and it might also have been nice to have been told a little about driver support, 64-bit compatibility, and impact of the OS on the performance differences.
Most people would be better getting a decent router (like a Linksys WRT54GS/L and sticking DD-WRT on it) for reduced latency, for $80/£40.
The End.