Greys are attractive character design. I really like looking at them, even though they are completely unsettling, when given the right set of features. There is a lot of talk about psychology, face recognition and biological directors for infant care that can be discussed for what makes the grey design so compelling, but I don't feel like doing any of it.
Aesthetically, I'd place them along side Nintendo's Mario and Eastman and Laird's Teenage Mutant Ninja Turtles as some of the most successful character/graphic design in modern history. This, however, only applies to the most modern version of the grey from the last thirty years. Earlier incarnations were missing a lot of what makes the model intriguing. Like Mario and the Ninja Turtles, the grey design is simple enough in form to allow for exploration in variation, yet specific enough in concept that any and all variations on the form are immediately recognizable.
The current aesthetic of the grey is so visually and emotionally effective that it stopped the majority of alien character design in our culture in its tracks. Before the concept of the grey was introduced, we were treated to all forms of green goblin and robotic terror. Following the first documented instances of the use of the grey aesthetic, the majority of all art, and even UFO case, that has anything to do with aliens or spacecraft utilizes some variant of the grey form.
Greys are a favorite of mine in the area of character, and even graphic, design. So much so that, like Mario and the Turtles, I have to avoid constantly pulling from the form when constructing my own designs. I'm a big fan of the Grey alien aesthetic and I think it will prevail into future generations, just as the other two designs mentioned have.