After playing the first three zones, I'm really disappointed in the game. It sure feels like the Genesis games, yeah, to a fault; there are a ton of minor flaws in the engine (like getting crushed between two corners of moving blocks) that could've been easily fixed but weren't. I'd hold left skidding and expect to turn when I went to spindash, for instance, which is far from game-breaking, but it would've been simple to turn Sonic around after a skid. The designers seemed too preoccupied with replicating every small thing to a tee, instead of looking at what could've been tweaked slightly for small gains.
If it was just that I'd be more than happy to overlook it, but the level design is what really let me down. I know the game shouldn't be too difficult in the first quarter, but I went through 90% of Chemical Plant and Studiopolis by holding a roll after a spring and watching a series of twenty-second roller coasters interspersed with simple gimmicks that ultimately boiled down to "touch object, go where the camera points maybe, and keep going". That kind of thing's fine for an act or so, but the third one in a row gets really boring. It's worse when almost every attempt to do something else felt like I had to fight with the level design to reach it, either through a weird slope jump or some other ledge that was just barely reachable.
The bosses tend to alternate between radical gameplay departures that have to be a snorefest in difficulty (or else they'd risk being an unfun wall to people who don't enjoy the different mechanics), and more normal bosses that, while fun, tended to get monotonous once you figured out the pattern. But then, I've never liked a boss in a Sonic game (or in most games), so I'd chalk that up to taste.
The special stages are really fun though, even if I don't quite have the controls figured out. Blue Sphere can bite me, though; the third time I ran straight into a red sphere from hitting the turn button a frame too late I gave up even bothering with them.
(Also the game's a glitchy mess. I haven't experienced too much of it myself, but my little brother's had a handful of softlocks and clipping through a platform to his death just from playing through the game normally.)
Like, I'll admit that part of my opinion is reactionary to the "first good Sonic game in 20 years" garbage floating around the internet and that I went into the game with a bit of a sour attitude from elsewhere. But I have yet to enjoy myself after the first three zones, so I don't see much point trying to finish the game to see what changes.