Thee is something that is both cool and terrifying at the same time about the Borg. I wonder which Star Trek writer came up with this brilliant concept that became the "killer app" for Star Trek: The Next Generation. Their implacable will and the stark simplicity of their spacecraft (why should space craft be streamlined?) makes them great television baddies.
They are flesh melded with machine, they return again and again, and they have a great catchphrase, in fact they are the Star Trek equivalent of the Daleks. No wonder, that they work so well.