They were really good. Of course they have to be mostly chained to their electronic instruments, but they made up for it with an awesome visual show.
They had a projector for the back wall, and then they had a scrim in front of the stage and they would project images onto the scrim that meshed or juxtaposed with the images being projected on the back wall. Most of the images were dark, gothic, macabre sorts of things. It was pretty cool.
They also had that cool series of LED columns that you can kind of see in the video, but in person were just awesome. Those LED colums can shine almost any color of light at any intensity; they're pretty frickin' awesome (I also saw, on a different night, Massive Attack, and they used those LED lights but on a much, much larger scale. It was AMAZING!)
And the singer left her post as often as she could to get the crowd dancing and amp up the club.
Having only found out about them one week prior (through my friend who got the tickets) I was really impressed. Perhaps because The Knife also play their slower, quieter, more atmoshpheric stuff as well, there wasn't as much of a sustained full rock testonerone techno energy as at say a Chemical Brothers or Underworld show, but, again, I was very impressed.
I would definitely reccommend going if you get the chance.