WOW - Why so much hate on Starlight! In my opinion she is an interesting character like the others and I like her presence in the show.
I think some of you are being overly critical. Those saying she has a bad back story with one pony being better than her making her want to destroy Equestria are wrong. The situation in her past happened when she was young and an event that happens in ones childhood tend to stay and deeply effect their future life; such as when people endure a traumatic event or something that shatters ones confidence leading to them never doing anything in life. This naturally let her to become a villain and rule a communist town. Then things went even worse for her when Twilight and the mane 6 threw away all she had worked for (even though it was the right thing to do) so she got even more distraught. And of course after years of negative thinking her ideas and views would be distorted to a point where revenge on such a large scale would seem viable to her. That's why she stopped when Twilight showed her that she was wrong and she can become reformed just like Discord was.
I think its interesting to have an evil character turn good and join the protagonists as its shows their character in a new and different way which is interesting to see. For instants the way she behaves around other reformed characters like Trixie, could she turn evil again? Also the funny little references back to her evil past are always funny to hear.
If anything Starlight serves as a refresher to the mane cast. Besides, its only been a few episodes, i'm sure they have more episodes planned for her in the future. Also, I don't consider her to be a new addition to the mane 6, shes just quite a big character at the moment; this means it will take longer for the writers to show us more about her.
As for Sunset Shimmer, well shes only been in the movies so I don't really consider her a character of the show.