This is an excellent game. I've been playing it the past 3 days on an older Xbox One and it's fantastic. I had tried the demo on our Switch, and that was actually very good as well. Our 6-year-old loves it on both consoles and says he does not see a difference so take that as you will... I do think it looks a little nicer and feels a bit more responsive on the Xbox One, and I expect it's as good or better on the newer consoles.
I had read a lot of reviews because I was surprised at how mixed the reception was in December; the price also dropped dramatically already also which gave me pause. No pun intended. But we're we've really enjoyed Generations and some of Unleashed (not to mention the many classic 2D Sonic games and of course the movies) so it was inevitable to try Frontiers. I really think if you like zooming around like Sonic, this is pretty excellent. The graphics, regardless of what people are grumping about in some reviews, are spectacular. For some reason the sound is really outstanding too, both in terms of the music and the effects. I am really surprised because we have an ancient pair of semi-decent computer speakers plugged into the TV but the way the sound is balanced and the quality of the score is as good as I've seen in a modern game, it's really really awesome.
People seem to groan a lot about "pop in," and I am aware of what they're talking about, but it really has no impact on my enjoyment of the game. Nearly all of the vast background scenery is there the whole time and just some small details "pop in" when you approach, from a distance. It is a non-issue, because, for example, you're still gonna see that rail you need to jump on way, way, way before you need to input the jump or dodge or dash or whatever. The gameplay really is fantastic - they balanced the insane speed with very tight controls considering the high-speed mayhem.
Some gamers aren't considering that the huge variety of challenges and control options exists so that there is appeal both to "elder" gamers like me AND really young Sonic maniacs like our kids. It's an impressive achievement because while there are plenty of "dumb" puzzles and easy enemies to battle, these are balanced with tons and tons of wild aerial challenges, many rather tricky, and some totally bananas boss battles. The guardians, which are sorta mini-bosses, are super impressive, fairly challenging, and just fun to deal with. But yeah the main thing is the zooming all around like crazy - it is amazingly well-done and if you at all enjoyed older Sonic 3D games, I think you'll be thrilled to have the freedom you do in this. Sonic was originally created in response to Mario, and it has always had many parallels yet a totally different feel. Today, we can see some strong design similarities between Sonic Frontiers and Super Mario Odyssey- the open zones, and the attempt to translate the feel of the classic 2D games into coherent, fun-to-control 3D games. I don't know if Frontiers will quite top Odyssey overall, as the latter was consistently a tour-de-force, but having only put 3 or 4 hours in I can say that Frontiers is just as successful in taking Sonic into the open-zone 3D approach. It's excellent, and I hope people will look past the many negative reviews to check it out - it has far exceeded my expectations.