Error opening iOS Simulator with iOS 8 beta version: “Unable to boot the iOS Simulator”

After I found some solutions on the web, a combination of procedures worked for me:

  1. Close Xcode and iOS Simulator (and all related applications)

  2. Install (or reinstall) Xcode6-Beta to Applications folder (direct from the .dmg file)

  3. Open Xcode6-Beta and go to Xcode -> Preferences -> Locations -> Command Line Tools and select "Xcode 6.0" (be sure that you've selected the one in Applications folder)

  4. Launch the iOS Simulator and go to iOS Simulator -> Reset Content and Settings...

  5. Close all the applications and restart your mac (I unchecked "Reopen windows...")

  6. Open Xcode6-beta and test the iOS Simulator

Good luck