Turbo switches - I say it largely depends. On certain games (especially on a pad), it's often annoying and doesn't really add any to the challenge of the game... However, on others, it actually feels "right" (especially on a joystick) to hit the buttons at just the right timing sequences.
Slow-mo - definitely. Considering that one of the primary challenges in video games is to react within a certain time to do certain things, artificially giving you more time to react has got to be one the fundamental definitions of cheating in video gaming.