Is Mary-Sue always to be avoided?

Everyone knows her. Most people despise her. I'm talking about Mary-Sue.
She's that character that is always one step ahead. All good characters love her. Bad characters may hate her but can't stop obsessing over her. She has no faults. Her heart is pure which is frequently mentioned by others. She's always the one to save the day. People look up to her because every now and then she drops a pseudo philosophical phrase. Any character of the opposite (and sometimes same) sex feels an amazing attraction towards her. With other words, she's everything we want to be. And she also has a counter part. Her antagonist is usually ignorant, ugly, uncharismatic and easily disposed of. Oftentimes he possesses certain traits of people that the author finds repulsive, like that arrogant bitch who gave you a hard time in school or the guy that would bully you after gym class.

While really good movies seem to stay away from a perfect hero (see my article Anti-Heroes) and focus more on a perfect villain (see my article Charismatic Antagonistic), I believe that Mary-Sue does have her place somewhere. And I think that place is among immature audiences.

Children can't appreciate a well-developed character as much as we do. For children it is important to easily distinguish between the good guys and the bad guys. Trash talk Mary-Sue all you want but she makes for a damn good role model. Would you want your child to imitate Hannibal Lecter? Probably not. So when it comes to stories targeting children I suppose the good guys should be good and the bad guys should be bad. Literally.

I'm personally not a big fan of the Twilight Series and I hear a lot of people complain about the heroine. She doesn't seem to have any specifically desirable attributes yet she is the object of affection/obsession of every other important character. So how can the series be so darn successful?
Who reads those books and watches those movies? Mainly girls in their puberty.
Girls who are self-conscious about their bodies and wish for acknowledgement from people who are probably too concerned about themselves in the same way (which is completely normal at that age). To them the heroine is a projection of their deepest wishes and fantasies. That is what makes the Twilight Series so compelling (to some).

Another great example for a successful Mary-Sue is Son-Goku. Who doesn't love Dragonball?
Son-Goku has a pure heart which is demonstrated by him riding Kinto-Un. In the beginning he seems a little stupid but in later volumes he is usually the one who comes up with all the good ideas rather than Bulma who is a genius. He is the strongest person in the universe and he's always one step ahead. Every one of his friends frequently talks about what a great person he is and he even manages to turn the majority of villains into good guys.

Dragonball is a shonen manga and the shonen genre targets young boys. So how come this manga is so successful even with adults? I think one major factor is that we grew up with it. The other factor is that Dragonball offers a variety of characters. My favorite character Vegeta is arrogant and shares some traits with a sociopath. But that is the type of character that mature readers are drawn to. Dragonball has a wide-variety of characters and I'm sure there's one for everybody.
That doesn't mean you should try to mix all types of characters into one story. It worked for Akira Toriyama (Author of Dragonball) but it is unlikely that a lot of people can pull a stunt like that.
Decide on a certain genre and stick to what the target audience likes.

It also can't hurt to find out if your character is a Mary-Sue or not. The best way is to ask other people in my opinion but if you're too shy or don't trust anybody's opinion, try the universal Mary-Sue Litmus Test.