I'm not super excited about the dub, but maybe that's just because kill la kill doesn't depend as much on over-the-top dialogue as, say, Gurren Lagann or Code Geass, so hearing the characters speak in a language I understand won't be as big an improvement.
That said... Satsuki's voice seemed a bit weak, but that might just be because it's difficult to voice a high-school girl who's also a fascist dictator. Gamagoori and Inumuta didn't seem to have the energy or enthusiasm that they did in the Japanese version, but the other Elite Four voices were spot on. Ryuko seemed unremarkable at first, but her dialogue got awesome when she pulled out the scissor blade. Shinji's voice seems really annoying, but she was kind of annoying in the original version too, so I can't complain too much. Senketsu's voice feels off somehow, but I can't put my finger on it, and Aikuro's voice was perfect. Of course, all this is judging from a few lines of dialogue each, so it's hard to tell how the dub will actually turn out. In particular, we don't know anything about
how Aikuro will sound when he drops his clothes and goes into full NUUUUUUUUUDISTO BEEEEAAAAACH mode, how Uzu will sound after he loses to Ryuko in episode 6 and decides to get serious, and how Tsumugu, Nui or Ragyo will sound -- Nui in particular is going to be difficult to voice well, and it would be disappointing to see such an entertaining character done poorly.