I'm kind of cynical when it comes to love at first sight, and I blame Shakespeare.
His definition of love at first sight, in Romeo and Juliet, was lust. And for the most part, he's right. When you're attracted to someone, very strongly as in R&J, it's usually about sex and/or infatuation. It's usually about the intensive emotional feeling when you first meet someone you like. Metaphorically in R&J, "love at first sight" doesn't end well, and causes a lot of grief.
For those that have stayed with their partners for years and years, and claim to have fallen in love at first sight, well... I think it's more of an expression of how much you love someone right now. I don't think anyone really knows if or how much they love someone based on appearances, otherwise that's pretty shallow, and therefore lust/infatuation. I mean, anyone who has been with a partner for years and years, can certainly say/justify love at first sight and say they're and example of it. But I think the years spent together has more to do with that, than the first sight.
But that's just me