"Sonic the Hedgehog" had four major things going against it. First, it's a movie based on a videogame, which are notorious for not understanding nor caring for the source material. Second, it's a live-action/CGI hybrid, which are generally lazy and uninspired. Third, it came out several years after the character had major relevance, which reeks of nostalgia exploiting. And fourth, the design for the titular character was terrible. It's quite telling how, against all expectations, it managed to push through every single one of these issues.
The most obvious is, of course, the character redesign. After literally everyone on Earth was horrified by the original design, they decided to change it. A lot of people think that was some sort of publicity stunt, but the reality points to the director using the negative feedback to change what was undoubtedly a studio-mandated decision.
As for the other points... I'll say it outright. This movie doesn't break any new ground, but what it does it does right and isn't content with delivering a paint-by-numbers product. If you've seen any other movies of this kind (such as The Smurfs or Alvin and the Chipmunks), you can already guess how the plot is going to play out. Unlike those movies, though, this one doesn't insult your intelligence.
There are a lot of familiar tropes in this movie, but they're actually well thought out. Yes, Sonic seems obsessed with current pop culture, but there's a valid reason given for it. Yes, Sonic goes into a road trip with a human rather than using his abilities, but there's a clever (though subtle) explanation for it. A lot of familiar territory is covered with attractive scenery as to make the trip more interesting.
The humor in this movie is actually pretty clever rather than the usual "it's for kids, and they'll laugh at anything" excuse (save for the inevitable fart joke, which is likely another studio decision), and the references to the source material are interesting and respectful.
Extra points have to be given to the villain. Villains are rarely the high points of these movies, but this is a major exception. Not only Jim Carrey's Dr. Robotnik is a proper representation of the original, he actually elevates him. And yes, he's bloody hilarious.
A couple of subplots are introduced and then completely ignored, but in general the story is well written and well acted. There are no annoying characters, which is extremely rare for this kind of film. And while Sonic'a new design is a bit more cartoonish than the first one, it meshes much better with the real world by virtue of acting and animation.
This movie is not particularly original, but it's certainly very entertaining. Parents tend to roll their eyes and groan at how awful these movies tend to be, but this time they're likely to be at least as entertained as the kids, and likely even more.