Much like Sonic Mania, after playing through this game for the first time, I came to the conclusion that it was mostly only fun if you were good at it. And I sucked at this game. But I don't think that conclusion's quite right this time, though (it's like... half right). Sure, Classic Sonic's stages were just flat out frustrating and unenjoyable for me most of the time, with enemies seemingly placed specifically for you to get blindsided, and much more waiting for moving platforms than I expected for a game about speed. But the Modern Sonic stages I found super enjoyable despite me not being so good at them, enough for me to hop right back into one right after finishing it a couple times. I'd played Sonic UnWiished and Sonic Colors before, but this felt like my first real experience with the boost formula (boosting in UnWiished worked a lot differently, and Sonic Colors is... well, Sonic Colors), and it was great. The levels have so many different pathways to go through, and it feels amazing to quickly pull off a jump and boost to reach a path you'd noticed before but couldn't get on, only for that path to take you to an entirely new part of the level you'd never seen before. This is the first Sonic game I've ever really felt motivated to keep playing after beating, and it's all thanks to the glory of Modern Sonic's levels. Those Red Rings are actually a lot of fun to find this time around.
Having played the game some more, I don't hate Classic Sonic, though I do find him a lot less enjoyable than his modern counterpart. The fact that you can't build up speed rolling down slopes kills so much of what made Sonic's classic outings enjoyable (though I've only played Sonic 1 and Mania), but at least the tweaked Spin Dash is fun. Unlike most of Modern Sonic's stages, Classic Sonic's do only feel fun when you're good at them, as the tedious platforming is reserved as a punishment for falling off the higher paths (which you're unlikely to be able to stay on your first time through the stages (or maybe I'm just bad at Sonic games...)), and enemy positions are no longer an issue once you already see them coming. Classic Sonic stages are perfectly enjoyable on like your third or fourth playthrough of them (and Crisis City Act 1 was legitimately a lot of fun for me to play over and over trying to get the S rank), but y'know... I wish they were fun the first time too.
The little challenges that you have to complete at least some of in-between stages I found boring during my initial playthrough (and they felt like padding, too... I just wanted to play more Modern Sonic stages, but I have to wade through this boring other stuff first?), but coming back for 100%, I thought they were mostly fine enough. Some are better than others, of course.
S ranks are pitiful, which removes so much of the satisfaction of getting them that other games had. It's less like “whoa, you've perfected this level!” and more like “oh, you didn't majorly mess anything up!”. It took me over nine minutes to beat Planet Wisp Act 1 on my first try, and I still got an S rank. I just wish they were a bit harder, as they'd be more satisfying to get and would likely give the game a bit more longevity as I replay stages trying to perfect them (and Generations definitely needs a bit more longevity... it's a short game).
Despite some frustrations, Sonic Generations ended up feeling like a pretty solid, if short, experience. It's held back by Classic Sonic stages that just feel less interesting than the Modern Sonic ones, and padding with lackluster challenges. But when Sonic Generations is good, it's good. Modern Sonic's stages are legitimately the most fun I've had with any Sonic game I've ever played. Sonic Team really nailed the boost formula with this game... and then, as we all know, things went a little bit downhill for the series. Sonic Generations ends up feeling like lightning in a bottle, a fleeting glimmer of the boost formula's potential that could never quite be captured again.
(As a side note, I did run into some pretty frequent performance issues (mostly lag and a some really annoying freezing that forced me to completely shut down my laptop, as the application wouldn't let any windows display over top it and task manager couldn't even kill it), but I'm pretty confident that that's not really the game's fault, and rather just that I have a really bad computer.)