Every writer needs a strong vision for who their character is to be. Even a character who has many 'Mary Sue' traits can be written well and interestingly by the right writer.
The important thing to remember, IMO, is that everyone who reads your story will have a different opinion of it, simply because we all look for different things in a story.
My usual guideline is whether or not I would want to read the story I am thinking of writing. Chances are, if you like your story, someone else will too <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
But the trick with that one is to be honest with yourself. It's hard to look at your own work with a critical eye, but it's something we all, as writers, must learn to do.
If you hate your own finished story, either keep it around as a reminder of what NOT to do, or delete it straight. But don't post it. Ever.