Finally got around to watching this, and it's good, but not as great as the opener and other two-parters. Starlight's motivation needed more time to be expanded upon, and I think the reform would've been more believable if Starlight had to face one of the villains, like Tirek or Discord, instead of being taken to a wasteland. It would've been a perfect opportunity for Twilight to show Starlight just how much her friendship with the rest of the mane six mattered.
Think about it: Twilight drags Starlight to the changed present. Starlight notices that it's different, but assumes Twilight is lying about just how bad it is. Then comes one of the villains. Starlight tries to fight and fails, then Twilight explains how she stopped the villain with her friends and takes Starlight back to the past. It wouldn't have taken too long and it would make Starlight's redemption better.