First off, the disappointing things: it's lame that Green Hill Zone is in it for a whole two minutes, where we get to meet Original the Character Longclaw for all of 30 seconds before she proceeds to never appear in the film again. Same goes for the echidna tribe, who are after Sonic's power for reasons that go totally unexplained. A copyright listing for the movie claimed that a "lizard warrior" was the foe after Sonic's power, so I can't help but wonder if these baddies were also going to be random new characters that were changed into echidnas to add a tiny bit of Sonic Adventure fanservice when the redesign happened. In any case, Sonic fans can probably figure out what's going to happen to these guys, so it bodes well for Knuckles' chances of appearing in the sequel. Also, they reference a generic mushroom world constantly throughout the movie, yet for some reason no one during production once said "hey, there's actually already been an iconic mushroom-themed world in a Sonic game, maybe we should just say it's Mushroom Hill Zone"? Weird.
What all this boils down to is that there's very little in the way of Sonic-y stuff outside the core basics of rings, speed, etc. Perhaps most distressing is that a series famous for its insanely good soundtracks has almost none of it on display in the film. You hear tiny snippets of Green Hill Zone near the beginning and end of the film, and during the actual GHZ sequence it plays the Sonic Mania intro theme by Hyper Potions, but other than that? Nothing. They were in San Francisco for a chunk of the film, you're telling me they couldn't have City Escape blaring? Junkie XL's score is fairly forgettable orchestral stuff as well, although Eggman's theme is kinda cool.
Now onto the good stuff: They really nailed Sonic as a character. I have to give Ben Schwartz a big apology because I thought he was going to suck based on the first trailer, but he absolutely knocks it out of the park. The movie actually has logical explanations for why Sonic is both ultra chatty and obsessed with the real super power of teamwork, I laughed out loud when Sonic is literally revived by friendship during the climax. Jim Carrey was also perfect as Robotnik. He isn't an exact portrayal of how he is in the games but they nailed all the traits they needed to. He's got the goofiness of modern Eggman with the menacing presence of SA2 Eggman - oh, and Sonic actually starts consistently calling him Eggman by the end of the movie, which I honestly didn't expect them to do.
I think the biggest surprise for me was that James Marsden's character is actually not awful. He's not even just passable. He's legitimately entertaining and enjoyable, and he has great chemistry with Sonic. I think they should definitely ditch Earth and stick to Sonic zones for the sequel, but I would be glad to see Tom come along for the ride. The other human characters were okay too, namely Maddie and Agent Stone. The only big dud was Tom's cop buddy, who had exactly zero funny lines the entire movie.
Speaking of funny lines, there are actually a lot of good jokes in the movie, and surprisingly most of them were not spoiled in trailers. The delivery is a big part of the enjoyment, even lines that aren't really amusing on their own are greatly elevated by the way they say them. Unfortunately there are also a lot of mediocre jokes that don't land, but thankfully I can't remember too many jokes that were actively cringey aside from a couple of pop culture references and product placements that are going to really date the film someday (what the hell is Zillow?).
And of course, we can't forget that redesign. I've seen the sentiment that this film would have ultimately been exactly the same without it, but... no. Just no. I can't even fathom what this film would have looked like with that creepy critter onscreen for the majority of it. So much of the movie relies on Sonic looking cool, cute, and even vulnerable, all things that Gremlin Sonic was physically incapable of expressing. The redesign was definitely the right move no matter how you slice it.
In short: kids will love it, Sonic fans will enjoy it, everyone else will probably think it's OK but nothing special. Personally I think it was a solid start to what should absolutely become a series. The second film is all but written for them - you've got Tails seeking out Sonic, you've got Eggman plotting revenge, you've got those tribal echidnas who've probably unleashed Chaos by now - my only request is that they lean into the Sonic stuff next time. We've done the Earth stuff now, so unless they're going to do a Sonic Adventure adaptation, let's make use of those ring portals and let Sonic and friends go to all kinds of familiar zones from the games.