I haven't been to the park yet to actually see it, so I watched it a few times on YouTube instead. Overall, I loved it. The few things I didn't care for were the new main vocals, some of Mickey's choreography, and that it didn't suddenly end like it used to. I worked in Attractions many years ago, and I worked Fantasmic most of the summers.
The voice of the old vocalist was a very soothing thing to hear after working several hours of chaos moving poles in the hot sun and directing people to go here or there. Even hearing that voice when seeing the show after I quit working at Disneyland, I'd still feel a wave of calmness come over me. I didn't feel that with the new vocalist, and I don't find her voice to be soothing at all. Her voice would be good for a daytime parade, but not a nighttime show.
I also loved how Mickey used to playfully wave his arms in the air at the beginning of the show. He no longer does that. I miss that. I also couldn't tell what he was doing on top of the cabin during the finale of the new show, as the cameras are panned out to show the fireworks, but it looks like he also no longer waves his arms to "direct" the fireworks. I guess I'll have to either find a video that focuses more on Mickey in the finale or go see the show myself once I'm no longer blocked off. But if he no longer does that (which it appears he doesn't, from what I've seen), that's another thing I'll really miss.
I also missed the old show ending, how there would be fireworks, a second or two of looking at the lit, empty island, and then, boom, everything goes dark as the last music note sounds. I don't like that it ends with fireworks, and it appears that the island goes dark and the music stops a second or two before the fireworks stop, but I could be wrong on that. Still, I prefer the old ending.
I'm iffy about the Pirates vs. Peter Pan thing. I like the Pirates addition way better. Watching them side by side, the Peter Pan scene almost seemed hokey compared to Pirates, but I think Peter Pan is a better fit to Mickey's dream than Pirates is. I also miss the, "Hey, uh, what's going on? Uh-oh," followed by the music, a second or two of silence, and the canon shot. I do like the replacement to it, though, Barbossa (?) appearing on the mist screen before the ghost ship comes. I love how they made the Columbia look really creepy.
The other thing I didn't like about the new version but isn't that big of a deal is that they seem to be focusing mostly on new characters, since Little Mermaid. They took out most of the scenes with pre-Little Mermaid characters and replaced them with newer ones. It's not the addition of the new characters I don't like; it's just I wish they would have found a way to keep both.
Other than that, I prefer the newer version to the older one. Love the upgraded special effects, and I love how they tweaked the old music (it sounds the same, but like they added more instruments to the old soundtrack). I can't wait to go see it in person!