Jagged Knife event is insane! I just had a game (gigantic 9 races), where Jagged Knife took 103 planets, 7 of which were mine. I had 19 at the time. I lost Man, research and eco capitols as well as 3 or four wonders   .
I tried to soldier on and actually took 3 or 4 back but I ended up starting a new game.

yea, they are like the I-League, only more aggressive. Like thier name indicates, they are a violent group.

The event is pretty fun, but depending when it comes up it can sometimes be overly easy.

I had it pop up in late-game and despite them having a lot of really nice well-developed planets it took about ten turns before they built a single ship. The moment they appeared I put military production to 100% and had every planet start building troop transports and was able to conquer half their planets before they even had a single ship up.

When they did make ships, they were all troop transports (1hp cargo hulls) armed with weak defenses (something like 1 in all categories, but with 1hp why bother) and weak guns (about 9 attack when the weakest AI had 25 or so on smalls and the strongest had something like 100). I don't think there was a single planet in the game they could have taken with those ships, so they would have been a lot better off building defenders to keep their planets from being steamrolled.

Aside from that and the crash bug (when negotiating with them if the deal turns red the game crashes) I do like the concept of the event. It REALLY does mix things up (especially considering that when it came up I had 3 turns left for an influence victory ) and I have a feeling that if they had lasted five or ten more turns they would have became a massive threat.

This event would be alot more fun if the jagged knives actually did something after they taken all those planets. As it is now I will just reclaim my planets with little or no resistance.