I liked this game quite a bit, but I see the issues with it. The boss fights are pretty rough, especially the endgame bosses, though I think people are really exaggerating their issues. Compared to the original games all ending on long, drawn out fights with zero rings and limited lives, I think this game's fights are way more forgiving, even with the occasional insta-kill attack that's clearly telegraphed and not too hard to avoid.
The music is a bit hit or miss as well, though at worst the soundtrack is just okay (except for the Sonic 1 style special stage, that track puts me to sleep). The only real problem I have is how inconsistent the sound is, especially when you're enjoying some of the Mania-ish Tee Lopes music only to get smacked in the face with boss music straight out of Sonic 4. The main theme is pretty alright I think, Crush 40 did a cover of it for the Sonic Symphony and it sounded great there.
The physics are just about perfect, more than close enough for me to overlook the slight differences. Saw some people complaining that you move too fast to see what's happening, but I never really had an issue. Then again, people always say that about Sonic and it doesn't really make sense to me, so maybe I'm just too terminally Sonic-brained for it to bother me.
Overall I had fun, but the game could use a lot more polish and it definitely shouldn't have been 60 dollars. Maybe some of the multiplayer stuff adds value but I never really touched any of it.

Really didn't expect to enjoy this as much as I did going in, but this game is just so much fun in a way that's kinda hard to describe. Maybe it's because I'm used to modern Sonic (or maybe it's because of the mods that I was playing around with) but I didn't have nearly as much trouble with the controls as a lot of other people did. Things like the pop-in and the occasional bug didn't bother me too much, after the first few hours I didn't really pay much attention to them anymore.
The music was fantastic, but it's a Sonic game so of course it was. I also thought the BOTW-ish world design worked better than I assumed it was going to, and I really liked running and jumping around in all of the little mini-level segments they assembled in each area. The cyberspace areas could've been more interesting, though. I think I would've preferred if they were more involved, had harder challenge goals, and if they weren't just the same three level themes we've seen over and over again.
I thought the story was pretty good as well, I'm not super familiar with Ian Flynn's stuff but he seems to just get all the characters really well, and he's good at finding ways of making the story fun and engaging without constantly doing bits and winking at the camera (though there were definitely more than enough "Hey, remember this Sonic thing?" lines.) I do kind of wish there were more off-the-wall setpieces and worldbuilding scenes like in Adventure 1 & 2, but I appreciate how focused the story in this one was.
There were also definitely a lot of parts that show how much more dev/testing time the game needed (I don't really think the developers are to blame at all, but it is what it is). Most of the challenges were either trivially simple or gave the player way more time than they would ever need, and I'm not sure if that was just Sega not giving the team enough QA bandwidth or wanting to make it as accessible as possible, but it really cheapens a lot of the impact of solving puzzles when there's no real challenge. Towards the end of the game there were also a couple areas that were completely inaccessible, in a way that made me wonder if there was originally more content planned that had to be scrapped/reworked to meet the holiday release window, but they did at least make the parts that were finished fit together pretty nicely.
Overall I think this game was overflowing with creative ideas, though a lot of it was held back either by time restraints or a desire to keep the game accessible. Every fight is unique and memorable, Sonic's moveset is super fun to mess with, and of course the boss fights are an absolute feast for your eyes and ears. If you're not used to the kinda janky modern Sonic experience you might have trouble getting into this game, but if you're like me and have terminal Sonic Brain, this game hits on just about every level.

How in the world did I play this for over 70 hours

Completed the RetroAchievements set for this playthrough. Overall a really solid set, good balance of difficulty without relying on too many easily missable achievements or obnoxious requirements, as well as being a nice excuse to see some of the little touches you might miss otherwise. I did have to start up a second playthrough for the secret seashells achievement though, but that was mainly due to the game locking you out of two of them unless you obtain them at a really specific point.
Playing this game again after the Switch remake reminded me of how much I enjoyed it, as well as all the things I didn't care for about the remake: the soundtrack (while mostly good to great) had a few tracks that felt like major downgrades from the original, the dungeon builder feature was way less interesting than it should've been, and most importantly they removed the photo album mechanic. I guess it makes sense considering the photos were only there for the Game Boy Camera, but it was really neat having all these little moments captured and highlighted.
I could play this game a hundred more times and I don't think I'd get tired of it. For a Game Boy game, the level of detail and the amount of actual worldbuilding this thing has still blows me away, and reminds me why to this day I consider it my all-time favorite game.