There seems to be quite a bit of hate on the the Killer NIC brand. The reality is that there are improvements that can be made in networking to handle packet prioritization in order to reduce latency.
Big picture is that by default packets are handled as efficiently as possible and they're stored in the network stack until it's deemed efficient to send them. Now this makes sense for large transfers...think of it like sending a bus after it fills up with people. But the thing that sucks about that method is waiting until the bus is filled up before it leaves.
Killer NICs attempt to send the bus more often to reduce that delay. Now obviously this is a classic YMMV scenario as each different use case may or may not find this beneficial. In the case of gaming where speed and latency are king this can be beneficial.
Let's say you want to stream that movie over your internet connection, utilize torrents, or download a game on Steam while you're gaming. Obviously you'd want your game to have priority for packets because it's the only thing latency sensitive in that scenario. This is where the driver and software for Killer NICs comes into play. It's kind of like sending all the gaming packets on their fast tracked bus while making the non-gaming packets wait for a full bus.
Let's not pretend that the product doesn't work...because it does and has been reviewed and tested multiple times to that effect. In the end you don't get any gains on large transfers but see slight gains in speed on multiple small transfers including some significant latency gains in some cases. (for example UDP latency of 53.1 microseconds to 90.5 microseconds of a comparable Intel NIC as evidenced by Tech Report)
However, the real question is how beneficial that is for you in the real world. There are many other places you're losing significantly more time than that in the gaming cycle. In my opinion something like the Killer NIC is really only beneficial after you've taken care of the input lag on the video side of the house as it's going to have a more detrimental performance on your gaming performance than the networking stack will. Likewise this only applies to your computer, if you have another user in the house you're going to have to dig into router QoS options or any benefit will be lost.
As for Killer NIC failures it's not the the hardware, it's typically been the software. The idea and theory behind the product is solid, what's gone wrong in the past is the software execution (no pun intended).