Figured i'd put in a quick review, as a big fan of both the classic and many modern Sonic titles, this is one of the best in the series, but it definitely has room to improve.
The graphics and such are largely great with just some off things here or there, there are a variety of bugs I ran into, most particular the music would cut out once in awhile and never resume until I would trigger a new level, the new music and the remixes are all very well done, fantastic boss fights that only get better upon repetition, and the best special stages in the 2d series once you wrap your head around them. The level design is excellent and likely the best in the 2d games (and thus, the entire series), and overall it's just a joy to play.
With that said, it does have a variety of bugs, and some issues. For one thing, making Blue Spheres the special stage for the poles is kind of lame, while they are fun to play they take too long and don't offer any reward for in game, which means once you beat them all there is nothing to gain, no rings, no lives, no shields, no anything. Blue Spheres should not have been included as part of the main game in my mind, and instead maybe just throw in 4 or 5 random sorts of bonus games (with a mixture of the ones from sonic 3 and knuckles). I would have preferred Blue Spheres as its own mode with badges to earn and such. Also, I believe the game would have benefited a lot from the red ring concept being added in, they inspire you to explore the levels and Mania has a LOT to explore. Make them a shared resource for all the characters so that you can explore each level fully one time, and in turn, have THOSE be the thing that unlocks extra content, not blue spheres. Also add in letter ranks for time trial mode so people know what to aim for.
Aside that, while I love the remix levels mostly, a couple of them are too similar to the originals and in general, I feel too much of Mania owes its identity to the older titles. I would hope in the future the team is allowed to flex more original content with only some remix, and not the ratio we got. The story flow and such is also much less interesting than what you see in Sonic 3/Knuckles.
With that all said, Sonic Mania is probably the single best release of the series for the 2D entries, only competing with Sonic 3/Knuckles when locked on, which I would likely still put a little ahead of Mania overall, but it's close. The fact it's something I prefer over any single Genesis Sonic is in itself a huge deal though, excellent overall effort. I hope if they get the chance to make a second game, they take consideration of things like the above. If they want to go even above and beyond that, give more unique content for each character.
If you've ever enjoyed a 2d Sonic game at all, you can't go wrong with this one.
