I had the original, still do actually in my secondary PC that rarely gets booted up.

I liked that you could plug an external hard drive into it.
And then run a torrent/download through the NIC.
Which supposedly would give your download as much bandwidth as possible, without taking anything from the gaming bandwidth; as in give it everything left over, without slowing the game down.

The Killer was supposed to reduce your game ping by 20-30ms, which even if it were true wouldn't really be noticeable in any games.
It advertised slightly faster load times, when initially entering the game.
Until getting an SSD, load times were not noticeably different.

My thoughts were basically... very small improvements (not sure if there were improvements), too small to really notice.
But an extra piece to get, if you wanted to waste money to have the best of the best, in every category.