Before I go into my thoughts on Shadow's side of the story, lemme go over what they've done with the original game first. 3D Sonic rereleases usually have a reputation of being worse than the originals. Some like Adventure 2 Battle and Lost World PC aren't seen as too major of a downgrade, while some like Adventure DX and Colors Ultimate are seen as broken, buggy messes that destroy the legacy of the original. I have my own thoughts on those games, but I won't get into them now. For now, just know that if you were one of the people concerned how the original game would be ported over, it's basically flawless. The graphics got a minor increase in fidelity but are otherwise unchanged, and the gameplay is just as good as it was in 2011.
As for new additions, the Drop Dash was added to the game, though the way it was implemented is a bit... inconsistent. Classic Sonic has it as an optional skill that's unlocked from the get-go, but that means you can't bring it into side missions and boss fights. Meanwhile, Modern Sonic just has it as part of his base moveset, so I don't know why Classic didn't just have it like that either. As for its usefulness, it's pretty useless for Modern Sonic since you can just boost everywhere, but Classic has a few more uses for it. It's not as fun to use this time around because of the way Gens' Classic physics work, but it's a fine enough addition.
A bunch of different Chao have also been added in the stages as extra collectibles, and some of those buggers are in some pretty well hidden spots. Sadly, there's no reward for finding them all outside of them just appearing in the hub world, which is a shame, since I feel like giving you an extra unlockable move for finding them would've been appreciated. Mainly a double jump for Modern Sonic. I've gotten so used to the double jump from Colors, Frontiers, the game that shall not be named, and even Shadow's campaign that going back to the wimpy little air dash is just throwing me off.
Alright, but there is one major aspect of the game that's been altered in a controversial way - the cutscenes. I feel like some of the changes are waaaaay overblown by idiots who just wanna use the game as an excuse to complain about DEI or something (please do not talk to those people for your own good), but that being said, the changes to the cutscenes are a mixed bag overall. Some of the new dialogue is an improvement over the original while some of it feels a lot clunkier, specifically when characters are making references to previous games. What is universally worse across the board is the redubbed voice acting. I dunno what was going on with the voice direction here, but it's really confused. Roger Craig Smith and Colleen O'Shaughnessey sound like they're just reading off the script, but the worst of it is Mike Pollock. The dude is starting to show his age, and it's really affecting his Eggman voice. It may be time to consider retiring him as the character altogether. I feel like if this was any other Sonic story I may be more up in arms, but... it's Generations, the game barely has a story to begin with so who cares.
That said, some slightly worse cutscenes in a game whose story I never really cared about isn't enough to take away from the fact that this is just Sonic Generations with some added bells and whistles. It was already one of the best Sonic games before, and now it's just more readily available. If you haven't played it in a while, I recommend not sleeping on it, even if you only really bought the game for the Shadow stuff.
Speaking of which, onto the Shadow campaign now. Despite it being called "Shadow Generations", it's really more of a follow-up to Frontiers, using the same engine and having the hub world be open zone. That said, the action levels are much improved from Frontiers' cyberspace levels, with far better controls and rather meaty stage length. The more recent games post-Unleashed have been criticized for their ever decreasing stage length, with Colors and Generations having stages that were still fun but not nearly as long, and Frontiers and the game that shall not be named having stages that last only barely longer than a minute, if that. (Lost World is the weird exception that goes too far in the other direction). Well, I'm happy to report that the stage designers have listened to feedback, with stages that last from three to five minutes at a time. The stages are all varied and have their own gimmicks, with the first act of each stage being fully 3D and the second act being 2D (except the final level which is 2 fully-3D acts).
Helping to make the stages unique are Shadow's abilities. You start off with Chaos Control which you can use after defeating enough enemies, and they get pretty creative with how you can use it to access shortcuts or create platforms from otherwise dangerous hazards. As the game goes on, there are also unlockable Doom abilities which are used as gimmicks for the levels. They're all pretty fun to use and help make the different levels stand out. The absolute highlight of the game is Kingdom Valley Act 1, which is just a thrill ride from beginning to end with some of the best platforming and most varied alternate routes in the entire series.
The boss fights are all pretty fun as well. They all put Shadow's abilities to good use and have varied and fun patterns. I would say Metal Overlord was my favorite of the bunch, but they're all worthy inclusions into the game and not a single one of them feels like "the one boss nobody wants to play", unlike the Time Eater from Gens proper.
As for the graphics, this is the best-looking Sonic game since Unleashed. The models are all high quality, the environments look great, and the lighting is very well done. For the first time, it feels like they're actually putting the Hedgehog Engine 2 to good use, which is something I never really got the impression with when it came to Frontiers and... the other one.
The story is short but enjoyable. It's mainly just Shadow vs. Doom, but the inclusion of Gerald and Maria help to flesh it out just that bit more. If I had a complaint, it's that I kind feel like Shadow's arc is kinda treading old ground, with him having to once again learn to let go of his anger and fight only to keep Maria's wish of a better tomorrow in his heart... that said, I'm well aware the trend for recent Sonic stories has been more "character rerailment" than completely new arcs, since their personalities before Ian Flynn took over as writer were very flanderized. Outside of that one complaint, however, I enjoyed the story - the cutscenes were well animated, there were some interesting twists and turns, and the ending was legitimately heartbreaking.
All-in-all, I'd say the Shadow campaign was a resounding success and a worthy inclusion to Generations as a whole. While I do wish it was a bit longer, it's also worth nothing that the campaign was a side game added to an already existing game, so with that in mind its length is much more justified.
All that being said, while I'm not without complaints - the redubbed cutscenes of the original game being the biggest one - I'd say on the whole that this release is well worth the purchase. If you haven't played the original game, it's a good chance to play one of the best Sonic games ever made but with added content that's just as good, if not even better. If you have played the original, then this is the perfect excuse to replay it, and the Shadow stuff is so good that it might justify the purchase alone. Either way, I was very happy with this game. It may just be the first time we can say that a 3D Sonic rerelease is objectively better than the original, and that's quite the achievement.