This won't have explicit spoilers, but it will have implied spoilers, so read at your own risk!
I personally didn't like this episode as much as I liked the original ending. That said, this was still a great episode and retained most of the important parts that I liked about the original episode 23. I'm slightly disappointed by this ending, because it feels like more of a cop out than the original ending was, and it doesn't cleanly end the series. I think that the cleanliness of the original ending is one of its strengths. This ending is the same as the original for about half of the episode, before it turns away to do its own thing.
WHY I LIKED THE FIRST ENDING MORE: The original ending felt like the one that the series was leading up to throughout the show. This made the original ending feel like it gave adequate closure. It fit in line with the development of all the characters throughout the series, and didn't vary from that development or the previously established plot. For those reasons alone, I think that the original ending was slightly better than this one.
WHY I LIKED THIS ENDING: This ending was still true to the spirit of Steins;Gate, and, besides changing the overall way the series ended, retained moments that made the first ending impactful. It offers a few new ideas that were absolutely relevant to previous plot and character development, but that felt a little too sudden for me to be happy with them. Additionally, I think that a different ending doesn't take much away from the series because it just feels like an alternate world line; so I'm not terribly mad at it for simply being different. The biggest thing I think it brings to the table is building off of the original episode 23, and building excitement for Steins;Gate 0.
The 4 sentences immediately after this one (which I'll put inside of [[brackets]] to make it easier to see when it's done) will be the heaviest implied spoiler, so skip it/move straight to my summary if you want to avoid even implied spoilers!!!!! [[While I'm fine with, and can appreciate, the fact that they chose a darker ending, I think that this ending suffers from the fact that it is such a shift from what happens throughout the series. It felt like there was a bunch of development that happened throughout the series that was just ready for a bit of a push to climax and falling action, but then character development in a different direction suddenly occurred halfway through this episode, which gives it a sort of whiplash and disjointed feel - a sort of build up to nothing. That's the primary thing that makes this ending slightly worse than the original, IMO. That said, the point of this episode *isn't* to tie off Steins;Gate - it's to lead into Steins;Gate 0, so from that perspective, it gets the job done.]]
SUMMARY:
Regardless of which "timeline" ending you prefer, this is still a great episode. Definitely worth the watch if you like the series, and I think worth watching immediately after finishing Steins;Gate, and/or immediately before Steins;Gate0 (since the point of this episode is to lead into 0). If you're watching it immediately after the original ending, you can skip to about halfway through, because the first half is the same as the original. If you're taking this episode as an alternate ending to Steins;Gate, it's the weaker ending of the two, IMO, but it's fine, and still interesting, as a lead-in to Steins;Gate 0 (not sure that it adds much substance to the Steins;Gate 0 overarching plot, though)!
