I have mixed but generally positive things to say. In order of Classic, Modern, CH, Tag Team, and then story and package as a whole. Expect spoilers, not on purpose but if I refer to something you haven't seen, yeah.
Classic Sonic is the weakest thing here. Even comparing his controls to Gens, he feels worse. His music all kinda blends together as Genesis twang to me, and he has the least amount of levels. The fact that he is is here cuz of Mania's ending is pointless. He doesn't do anything that only he could have done, like say, share knowledge of his experience with the Phantom Ruby, or even MEET Infinite, other than being in a crowd scene near the end. What he accomplishes in the story could have been done by any of them. That being said, Tails bounces off of him in a cute way, and his levels aren't impossible to get over with.
Modern is like he was in Gens, with some changes. Double jump is obviously back from Colors, but he can't wall jump, drift, and he can slide but the levels very rarely ask you to. He can however maintain boost midair, be it from a jump, running off the platform, or midair, he stays in boost until you let go or hit a wall or run out. Biggest problem with him is his jump controls very heavily and the double jump isn't as helpful as in Colors, you'll make stupid mistakes over many precise jumps. That being said, his levels, while short and heavy on the log stretches of boost, they work fine and are fun to play. They have more alternate routes than people will admit.
Custom Hero is fun. You basically have 3 different looks for each species' hairstyle, 7 different eyes, no tail options, but plenty of colors to pick from for body, skin, eyes, and base bodysuit if you're a girl. You do however have a LOT of options for clothing, but at the same time it's oddly limited. For every different bit of clothing, there are 2 others like it but with different colors and you'll make a few good looks, but there's not a lot in the way of casual clothing. Body items can sometimes be shirts, and Bodysuits can be worn underneath, but there's no dark blue to be used as jeans and anything short looks like super tight booty shorts. As for the shirts you get, there isn't a ton of variety for that, so if ur OC is dressed kinda like how a human would, prepare to get creative. While it is limiting, it's probably for the better, cuz my OC's design got way cleaner after I made him in Forces. As for gameplay, there are 7 different Wispons. Burst Flamethrower, Lightning Whip, Cube Hammer, Asteroid Gun (that functions like the mechs in SA2), Hover Fan, Drill Drill (dunno what to call that), and Void Black Hole Gun. They all have their use to them, but how they handle your movement when using them ranges from a dead halt, to a lurch forward, and less control over your speed. Plan accordingly. As for using the Wisps, you can only use what you have as your weapon, but the capsules sometimes change to accommodate you, like if you use Hover, Burst sometimes gets replaced with Hover or is there alongside it as they both let you travel through the air. Wisps can be used anywhere and don't replace the stage music, this is a nice evolution on the formula. The CH has some generally good stages by combining this with Modern but without boosting.
Tag Team levels are few and far between, but you basically have access to both Modern's Boost and double jump, and the CH's Wispon. They share the homing attack, stomp, sidestep, rail grind, so doing the Boost or Wispon is what switches who you control. None of them are terrible and the one near the end in the Metropolis area is amazing, even if it is simple.
As for the story, it's fun to watch, everyone is at their best, Charmy is actually the best he's ever been, the kid learned to chill. Sonic has some great characterization, and Infinite is a good villain, until you play Episode Shadow and see him lose his shit over getting beat you by Shadow that one time. Becomes a hypocrite if you read that Forces comic where he abandoned his squad cuz he got a taste of power and then later says you can't trust anybody but urself when HE'S the traitor. Problem is cutscenes are short and it doesn't answer some questions you might have. How did the Phantom Ruby come to be? How did it get there? Who fixed Omega? Who made the Wispons? Where are Cream and Blaze? What was Shadow doing for 6 months? Did Rouge know? If she did, why did she not speak up on the fake Shadows? Why did Shadow not speak up about Infinite's power? Is Infinite dead now? I have no clue! I hear more free DLC is coming to build it out, so here's hoping that it's good. The music has some gems here and there, but some of it blends together in my memory when I'm not playing. Missions are basically achievements to reward you with more clothing and Wispons, but some of them unlock more achievements as well, often becoming "Do this thing, now do this thing a few more times even though you already did it that amount of times but this achievement wasn't unlocked yet so it doesn't count". After you get all 5 Red Rings in a stage, going back unlocks 5 number rings from Lost World, get them in the right order. After THAT, 5 Moon Medals (nothing like the ones in Unleashed) that you have to get all of in a short amount of time. Red Rings unlock more side levels where you do a short little challenge of platforming around some gimmicks from Colors like the green disco shapes that shoot green energy, or the yellow blocks or the bomb blocks, but so far, the number rings and moon medals don't do anything, not even achievements, tho you do get trophies for them on PS4 and Xbone and PC, but I played the Switch version.
Overall, this is not the best Modern Sonic can do. The level design team here is 1 Lost World Dev and 2 newcomers. If this had the Gens team on it, it would be amazing, but it isn't, and as a result, we have short, beautiful, railroaded levels full of enemies to mow down and long stretches of road. If you liked Rush and don't mind some simple mindless going fast, this will be plenty of fun for you. My first run was pretty sour but now with my 100% save file, I enjoy making characters and running the stages with different music playing in my headphones. Soaring Together from NiGHTS Journey of Dreams while running Nullspace, Up-Set Attack from Jet Set Radio Future in Capital City, Rivers In the Desert from Persona 5 in Arsenal Pyramid, good stuff. Also, I love love love the look of the Metropolis area (completely unrelated to Metropolis of Sonic 2 and Grand Metropolis of Heroes). The colors, the clean futuristic city, the music, it's my cup of tea and more. I'm gonna stop ranting now. My hands hurt.