They're biomechanical sex incarnate, what's not to like?
Seriously though I think they're a personification of sexual stigma, the fears of pregnancy and bodily harm, skulking around in the dark murder/raping people, quite simply monsters straight out of a nightmare, and yet we kind of identify with them, it's a common theme in the Alien movies that no matter how bad these creatures are the people who want to use and control them are worse. So in a sick twisted sense they're actually endearing, we want to see the Alien do as it does because in some voyeuristic way we feel liberated by it, the Alien is a raw representation of our repressed naturally selfish sexualities, that's why we love and fear it, because deep down we're all Aliens too.
We admire its purity.