I thought it was excellent! Very entertaining and exciting. When the first episode flashed up, "To be continued..." I actually was shocked and said to my daughter, "Already?" It just whizzed by.
Some thoughts:
One issue that some people are having is Starlight Glimmer's reason for hating cutie marks. I actually think there has been a lot of work done in previous episodes showing that the loss or withholding of friendship can be devastating. See: Party of One, in the first season and What about Discord in season 5. Friendship is shown over and over again (in other episodes also) to be very important to a pony's mental health and so it is easy to see why SG would go slightly off her rocker when she feels it has been taken away from her.
SG's reformation was quick and I agree that, as usual, the wrap-up was too fast--but, hey, they got in soooo much in just 44 minutes! I do think that she was totally unaware that her actions were causing such havoc in other timelines and that she was shocked when she saw the end game. I also think that it was great that she decided to stop on her own. It didn't bother me when she relented.
Starlight Glimmer's future: I thought that the montage at the end showed SG back at the Equalist town so I figured that she would stay there. I can't see her joining the Mane Six but I suppose it is possible.
All in all, a complex episode. A viewer would really have to be seeped in MLP lore and history to really enjoy it. A novice watcher would not really enjoy it on the same level as a person who has seen the entire show from Season 1. Is that a weakness or a strength? As someone who as seen every episode (at least for Season 1-4) many times, of course, I was thrilled with the references and call backs.