I have mixed feelings. I suppose it was altogether a rather good experience, but I wasn't really emotionally invested in this last run. Just for reference, my run was Neutral->Genocide->Pacifist runs. Genocide was a really nice change of pace, because although you spent more idle time fighting monsters over and over again and waiting for that random encounter, you wasted almost no time with puzzles and other dialogue. By the end, you don't even waste time with most boss fights, even. With the exception of Undyne and fricking SANS of course. I think I had the most fun with it, honestly, despite what it might imply about the players' psychology for liking it. It also had the most interactions with Chara, who I find the most interesting in the game. I'm going to refer to both Chara and Frisk as 'she' simply because I get this whole Madotsuki from Yume Nikki vibe from them. Chara was the most mysterious of all. She was initially portrayed in an innocent light, but was later shown to be far from it. In Genocide route, she seems to be some kind of omniscient, powerful mastermind behind the whole thing sans the war between humans. It's kind of like how in twisted stories, you end up liking the one innocent character, or once innocent character. In this case, I find the one twisted character the most interesting in this fluffy setting.
I found Alphys to be a really annoying character with her constant phone calls during the Hotland/Core sections of the game. Every 3 seconds it would be a phone call about something stupid, something incorrect, something misleading, or some update on her blog that I didn't care about. The True Lab somewhat redeems her, but I liked it more for the plot significance, explanations and for its surprising darkness rather than as something contributing to her character. It's also like the one place where we get to see how Chara and Asriel interacted together.
I suppose the True Pacifist ending was more or less what I was expecting, subconsciously, it didn't catch me off guard. Probably also because I had already spoiled the Asriel fight for myself. That it existed, anyway, I didn't watch any footage of any scene in the game, besides that ones Dunkey used in his video. All nice and twinkly. But had to add that little bitter-sweetness in how Asriel got his redemption but would be forced to revert to a flower again. Again, nice and all. But I wasn't emotionally invested in it. Genocide route was so new, so dark, and so... full of pure ah, how to put it, black malice, that trying to be all warm again was asking perhaps a bit too much. Or maybe not enough. It's easy to slip into the shades, but being asked to stand in the sunlight is really bright and hot. It was such a cold-hearted route. It didn't ask for anything. In some ways, it was the devil inside everyone, the one standing on one side of everyone's shoulders. Enticing you to give in to simple pleasures. All I had to do was play the game like I would play any normal RPG, killing monsters and gaining EXP. Gaining new levels, acquiring money. Watching numbers and counters rise, simple pleasures. Then the townspeople fled, and you could steal from the shops. Everything was accessible and open to you. The unthinkable, the amoral was so easily done. So how does one feel like, standing on top of the world, above the consequences? Kind of like in GTA. You steal cars and bikes at first, then you start running people over. That gets boring, so you gun down everything in sight. Dirty deeds done cheap. It was easy, really. All you had to do was fall. Fall into the pitch black abyss. Pacifist route asked you to spare everything, to turn the other cheek, to relinquish control and hope for the best in all things. It was certainly harder. I died a hell of a lot more for sure. Perhaps playing this route last may have affected my feelings for it. I didn't really care that much for the characters. In fact, I had a kind of Flowey moment. I was conscious that this was just one route, one possibility. So this is what these characters are like in this happier route, so I was thinking.
I dissociate Flowey and Asriel as a character, which is appropriate, even Asriel himself asked you to do that. I felt more sad for Flowey at the end of the Genocide route than for Asriel at the end of True Pacifist route. He was just begging you to not kill him. But more than that, he seemed more to be begging to keep him by your side, to recognize him as a friend. Since he developed his ideology, kill or be killed, from his experience with the human villagers at the end of his original life, it seemed he didn't even consider that it would apply to him and Chara, who both experienced the same violent scene and who were always together. It was tragic. Not being recognized by a friend as a friend. I felt more sad at that than the bittersweetness of being forced to return back to a soulless container, unfeeling and untouchable. Because the latter will never happen to me, while the former could very well be a possibility in life. It's sadder, because I can empathize with it more.