I can't see how it's worth it for an MMO. The majority of network latency that you're going to experience is happening outside of your home network. I think that the money would be better spent on upgrading your cable/dsl/whatever connection. The only thing that the Killer NIC does on your home network is something that most routers today can do, QoS - use it to throttle specific types of traffic (like torrents/downloads/whatever). The situation I can see that it might be useful is an FPS-game LAN party, where you might actually receive a minor advantage with a slightly lower ping, and your traffic isn't hitting the internet at all.
From the section on WoW in the linked article from 2006 (the closest analogue to EVE): "We see about a 4% increase over our other products in frame rates. WoW is basically capped at 64 fps with dual core systems so it is difficult to determine if the Killer NIC would provide any further frame rate increases with our test platform."
They also saw a max 10 ms decrease in ping, but...that's still below the human threshold of response. Going from 500ms to 250ms would be a big deal, the drop they saw from worst case 128ms to 119 isn't.
I can't see paying $100+ for a
possible 4%ish increase in performance.
Other anandtech articles:
2010 -
http://www.anandtech.com/show/3716/bigfoot-networks-announces-3rd-gen-killer-nic-killer-2100 - promised a full review within a week that still hasn't appeared.
2009 -
http://www.anandtech.com/show/2797/1 - page 3 & 4 - "Our EVE test is in a place where there were a very high number of other players and we were able to eliminate as many other factors as possible from testing. This test showed no difference in performance with or without the Killer Xeno Pro" and "In trying to do the similar testing with Team Fortress 2, the Killer Xeno Pro would be faster in once instance and slower in the next. There was no real consistency to our data in this case." and "In other words (and to sum up), when you have a bad connection, the Killer Xeno Pro is not going to fix it; when you have a good connection, the Killer Xeno Pro is not going to make the experience any better. "
A couple months ago, I read a blind study of the Killer NIC vs. a simple onboard, and the results indicated that people couldn't tell the difference, however I haven't been able to find that article since...I'll keep digging, though I might have to get back to you with that later.
Not the article mentioned, but further reviews:
http://www.extremetech.com/article2/0,2845,2037169,00.asp
http://www.overclock3d.net/reviews/misc_hardware/bigfoot_networks_killer_nic_killer_k1_network_cards/1
Again, I'd suggest either upgrading the connection from the ISP or upgrading other parts of the system first.