The Jagged Knife are a PITA but I eventually have a good experience from their appearance. I tend to grow slower than the AI in most games, so they don't hit me as hard as some other empires. As mentioned previously, the JK are a doormat to invasion.
IIRC the JK get all the current tech of all players when they appear. This can be a nice extra, and maybe even more so in TA. This means if you are a little behind in tech, or if the AI has discovered enough unique techs, it is a real opportunity to not only pick up some extra worlds, but to also pick up some free techs. So I rarely just take back my own worlds. I snap up as many as possible, because if I am ahead in the tech race, it gives the AI a chance to pick up some of my tech.
So as a whole I welcome the JK and the extra tech I will soon acquire. Ourobourous, AFAIK, one appearance per game. I have never had the Jagged, Jagged Knife!