Because I'm an associate editor, which is mid-level, I don't have an assistant, so slush does come directly to my hands. That's only if it's addressed personally to me though---any unagented submissions that simply say Ace or Roc on them go to our editorial assistants, the esteemed Danielle Stockley and Cameron Dufty. I also check the inbox we've set up for email submissions (sff@us.penguingroup.com). If you want to submit there, you should check out our guidelines.