I have one of these in my desktop at home and I have to say that it has made a big difference in my latency. These NICs are especially useful if you're multitasking on your NIC (Example: Downloading torrents while playing games). Money is only better spent if your machine is dedicated to gaming and gaming only.
I, for one, do much more than just Game on my computer. I download movies, music, applications. I host ftp sites, web servers, and services for use on my local network. The Xeno assists in many of these tasks and keeps my ping low while my family uses services hosted on my machine. I don't have to purchase a new machine for hosting these services, and I retain my good ping times. That's a win/win in my book.
As stated above, there are reasons to want after-market NICs, but the bang for buck just isn't there if all you're doing is gaming. I feel most users that criticize Killer NIC simply do not see their computer for anything more than a gaming machine. Do yourselves a favor and think outside the box.