Right now this - the video below - is a one off experiment.
But it's still one human controlling another by thought alone. And that gives me the screaming heeby jeebies.
True, it's literally just getting the other guy to twitch his finger. But it shows that, in principle, one person could get into another's mind and hijack their body - zombifying them in a way.
But no-one can deny the potential of technology like this, and the good that could be done by developing it further: Watch this quadriplegic lady feeding herself, using a machine she's controlling with her thoughts: