Undertale Spoilers Herein

Pro 1 - The visual presentation was almost spot-on, I don't know how else to explain it. The only thing that I'd pick on is how slow Gaster's attacks are slow without a mechanical need for it, specially since many of their components use the whole screen, so you see things coming at you form a mile away. That said, the 'proper' slowdowns when there were too many things in the screen was a great subtle touch I loved.
Pro 2 - The story element in it was very well done, but it's my sincere wish they tried another theme with it. We already worked it with neutral and pacifist Floweys, no need for another visit on this particular 'feeling of helplessness' theme.

Con 1 - The music used here was a bad choice. It was too 'thriller', not bad but inadequate for what this is: "lunatic going even more insane by their power as they lunge after you... but this is a videogame". All they had to do is look at Earthbound 1's Giygas theme for the last fight. I mean, it even stops being music at some point. The thriller music, even the 'corrupted' version made this cheaper than it really should IMNSHO.
Con 2 - The ending is more meta than it needed to be. I know they felt the need to up neutral Flowey's taking over the save and Chara's savefile corruption meta, but there was never a real need for it, which cheapens it further in my eyes. To make things worse, it's just a 'up to eleven... done wrong'. If Flowey's thing was a Micheal Bay explosion, then this was just a slighter bigger Micheal Bay explosion which doesn't work as well because we already saw Flowey's one.

What I would have done different:
I would have changed the music first, duh. Then I would have worked the 'Gaster has become something so alien that nobody knows what's going on' side of things.
Just imagine how the whole battle was just Gaster attempting to communicate and damaging or healing frisk in the process. Imagine is Frisk's attempts to mercy Gaster would ultimately end up in Frisk's death, and a more violent approach with a few ACTs here and there could allow Gaster a brief moment of clarity to communicate what they wanted before leaving.
From there on game would have glitches in a few select places until the next reset. But Gaster wouldn't be there anymore in a new game: they already said what they needed to. Attempting to unlock the Gaster's apparition would only make the glitches return. And that's how I feel the meta should be done here.