Personally, I hate Mary Sues. I define them as overly perfect, overly powerful characters who have no drawbacks or consequences to said power or perfection. They are, what those of us in the trading card game world would call, 'broken'. They can beat any canon character at anything, and every canon character just adores her for it.
However, I don't think it's fair when an OC is automatically labeled a Mary Sue. Granted, for every decent, well developed OC there's usually about a dozen more Mary Sues just waiting to rape whatever canon they set their sights on.
One of the arguments I've heard is that if the OC is involved in a romantic relationship with a canon character then they're a Mary Sue. Personally, if you can find a way to reasonably place and OC with a canon character, without making the canon character OOC, then go for it. Some canon pairings are just cringe worthy, so if an author wants to mess with that in a believable fashion, I don't see the problem with that.
Ok, my rambling is done.