It looks...mediocre to be honest. I haven't played it yet, and I don't plan on buying it at its current price; $40 is too much for a game that lasts maybe 3-5 hours.
My opinions on it are incredibly mixed. I do like the Unleashed style Modern Sonic returning, and the custom character is a neat gimmick. The graphics look very nice for one minute, but turn bland and boring the next for most stages. That's about where the positives end for me.
The new music direction isn't really my cup of tea, except for the main theme, "Fist Bump". That's a great track, but there isn't really much else I remember except Classic's Chemical Plant copying Eye of the Tiger, and Classic Green Hill literally giving me a headache. Speaking of Classic Sonic, why did he have to return? I know that the Phantom Ruby transported Classic to the Forces dimension, but he was already in Mania, so why butcher his playstyle and shoehorn him into this game, as well as Shadow, Chaos, Metal Sonic and Zavok? Also, didn't Generations establish that Classic was from the same dimension as Modern Sonic, but in a different timeline?
The story also appears to be confusing in some areas; when Eggman describes the Phantom Ruby, it sounds like he made it himself, yet in Mania's intro, it shows Eggrobos finding & pulling the Ruby out of the ground. Also, while all of the worrying is being done about the Phantom Ruby and what Eggman & Infinite are going to do with it, why doesn't anyone think about getting the Chaos Emeralds? Those gems never fail to stop Eggman's plans, so why not use them here to make the plot and game more interesting, and while you're at it, throw in an epic final boss?
Finally, Sonic is killed by Infinite after completing Green Hill the 1st time, but is, mere minutes later, found at the Death Egg still alive. First, this brings back bad memories of 06 and Rise of Lyric, where the exact same thing happens. Second, why would you build up all this drama about Sonic dying, Eggman ruling the world and Tails "losing it" when you're going to throw it out the window so soon? Also, if anything, Sonic's friends have better abilities than him, and should be able to take out Eggman easily.
Remember when Tails took out Chaos 4 and Eggman all by himself in Sonic Adventure? Well, now he's afraid to even touch Chaos 0 and wants Sonic to rescue him.
So, that's my beef on it; I think I'll just stick to Sonic Mania for now. If I ever were to review it, I'd imagine it being around a 50-70/100.