Well, I can't say this episode was one of my favorites, but it definitely was better than I expected.
I'm VERY glad that both Spike and Starlight didn't turn out to be simply mediocre characters like they have been for me in the past. Spike was always the "great guy who's mistreated or misused", and to me Starlight was the worst character going in. I really disliked her cliche "rough backstory excuses bad behavior" in the Season 5 finale, but seeing her have to cope with her own guilt is... well, a bit better for me. She's a LOT like Sunset Shimmer and Luna in this regard, and I LOVE character arcs like this.
As for Flurry Heart, I... well, I really hate her. I know you can't really blame babies for doing bad because "they're babies" and "they don't know any better", but this is just awful. Let me explain my points:
Hasbro made her an alicorn right off the bat. And when this was brought up, when Rarity asked "Shouldn't alicorn wings be earned?", the others were all "Oh, well she has wings so who cares and why ask?". To borrow a line from FluttershyFiM, this episode does an excellent job of addressing then promptly ignoring its own plot holes. So she's an alicorn, was born into a royal family, and has OP magic and OP flight. She's my worst nightmare come true, a bad OC implemented into the show's canon. Fortunately I think they've put a suppressor on that weapon so she might be a bit less of a destructive force, but still. She was the ONLY plot-starter for the entire episode. We don't get two-parters very often, and when we do, I expect there to be an interesting and dynamic conflict which has an interesting solution, but in this case, it's a deus-ex-bad-OC conflict and a deus-ex-Sunburst ending. You can't really rat on the ending because they kinda had no choice, what with bad OC alicorn destroying everything, but the start was just horrendous.
Overall, while I don't hate this episode, I still really hate Flurry Heart. I just hope that a] whatever magic or flying ability remains suppressed, and b] that she simply becomes a prop for Cadence or Shining Armor to show up with.
As for Sunburst... I really like this guy. He's a male character who's not only helpful, but has an interesting (albeit rushed) character arc, he's NOT a plothole (and I don't mean a fluctuation in the story), and he's not stupid. He's better than most male characters, and I think I like him even more than I like Shining Armor.
Speaking of Shining though, his stress is amazing! It looks like stress runs in Twilight's family, and I hope to see more of this explored in the season. Just a side-note there.