I'm gonna try to be as fair as I can. Sonic Classic is a semi-original 2d genesis style fangame that primarily takes inspiration from the 8 bit sonic games. Its special stages are heavily based on those of Sonic Chaos, its levels either based on or outright ripped from game gear titles, but its level design is completely original. It starts strong; the physics aren't too bad, the presentation looks really nice for the time, the soundtrack is fine, and the early levels are really fun. Once you get to Gigalopolis Zone (which is ripped from Sonic Chaos) though, everything falls apart. The level design is flushed down the toilet, and once I got to Luster Lake Act 2, I became softlocked, as no underwater bubbles would appear to give me air for the entire level which is almost entirely underwater. This completely game breaking bug on its own is enough to bring the game down to half a star, but by the time I got to that level, I already wasn't having very much fun. It devolves from a semi-original and well designed game to a compilation of "worst of 90s 2d sonic".

Bosses and soundtrack are good.
Otherwise this is a step down from before the sequel on all accounts in my opinion. The level design is significantly worse, with less speed and more poorly implemented platforming gimmicks. Enemy design is even worse. Special stages are still awful (albeit still totally pointless). Aesthetics/theming are a step down. Cutscenes are longer and more cryptic.
I don't recommend this game at all, but if you absolutely must play it, avoid after the sequel "omega." The widescreen upgrade is tempting, and it does touch up the dated sonic worlds physics, but it introduces a million of its own problems instead, including overtuned boss fights, horrendous loading times, laggy cutscenes, and tweaked level design that makes the already bad levels even worse.

Very charming and original, even to this day. Level design shows its age in that there are lots of automated bits and the platforming is very vertical and rudimentary, but fortunately there is very little waiting. Physics don't replicate the genesis but they're perfectly serviceable as their own thing. Bosses are surprisingly good, and cutscenes, while dated, are brief and enjoyable. The soundtrack is very good.
Biggest complaints include terrible and pointless special stages, an insultingly easy super sonic final boss in which your rings do not drain (I think this was added in Plus), bad enemy design, and some gimmicks that, while great ideas, control poorly (such as the Yoshi's Island fuzzies in Star Shores Zone and the gravity reversal in Arcane Altitude Zone). I also just hate Cloudy Crowd Zone in general.

After playing a truly terrible 2D Sonic game (Advance 3), I came back to this game hoping I could appreciate it more. Sure enough, I can. I used to hate it but I finally see the light.
Starting with the positives, the presentation is excellent all around. The soundtrack slaps top to bottom and the art style is very well done. At first I thought that this cutesy look isn't right for Sonic, but it eventually ended up growing on me. I was right. It isn't right for Sonic as I know him; but this isn't Sonic as I know him, it's Fallen Star.
The special stages are very challenging even after the patch if you ask me, but I warmed up to them significantly. After overcoming my skill issue I realized that the fact that they're littered with hoops and springs that can fling you into the pit at the right speed is in fact the whole point. Being able to judge at a moment's notice what special stage gimmicks you should and shouldn't interact with is a skill that you can develop through practice; and eventually I did.
The theming is fantastic. Taking CD's penchant for alliteration and using it to blend many aesthetics and ideas into downright surreal combinations that give Sonic CD a run for its money. I mean come on, carnival crater??? What is this, Rollercoaster Tycoon? It's awesome.
Unfortunately I still have major misgivings with the game. For starters, the level design is all over the place. It has a well earned reputation for being extremely easy and borderline automated, and yep, it sure is. In terms of player input this isn't far off from being a 2D Forces; at least until the last 1/3 of the game where some more platforming oriented gimmicks are introduced (and in thunder turbine's case, super mario maker hot garbo levels of enemy spam. yeah that zone stinks).
The bosses are pretty terrible. idk why sonic fangames consistently have issues with this but they range from cryptic to boring to braindead to what the actual fuck. These bosses really just fall literally everywhere on the spectrum of bad bosses. And the funniest thing is that the boss of the first zone is one of the hardest so lol get fucked
Lastly I have to criticize the cutscenes because my god they are so boring. Nothing happens in them. There are some neat interactions between sonic and tails every so often but they are dragged out so long and communicate a paper thin story. Just skip them.
Overall this is an okay game imo, it isn't on par with greats like time twisted and triple trouble 16-bit but if you played those and need more then go ahead