Well, somebody's got to like Mary Sues, or else they wouldn't get published and the books wouldn't sell. They do, and they do. When people get together and snigger at Mary Sues, I think part of that outburst of laughter is the embarrassed recognition that one has loved, and continues to love, various authorial ego-ideal characters despite their obvious tricks.
The other part seems to me a venomous judgement of the ordinary fantasies of women -- who doesn't want to be perfect and talented and loved in every way? I think readers -- especially readers with fingers on the critique pulse -- are crueller to female Mary Sues than to male ones, maybe because 50 years of crap adventure have inured the reading public to perfect male heroes. But maybe the virulence of Mary Sue hatred is related to the intensity of love/hate of celebrities like Jennifer Lopez, whose life is a storybook romance, until she is suddenly a punishing diva ho.
I think I just argued myself into the idea that Mary Sue is a feminist issue. Huh.
Anyway, my thinking about Mary Sues began to change when I found out they used to be the (a) most popular and (b) highly prized genre of fan fiction. It's only recently -- say, ten years -- that people have begun looking down on Mary Sues so consistently. I look down on them generally, because they tend to be unsubtle and clumsy, but I've finally come to the conclusion that they're harmless.