I'm a bit torn on this one.

On one hand, this is my favorite Pokémon game in a decade. In terms of structure, it's exactly what I've wanted from the series since it went to 3D. Exploring a region and raising my Pokémon at my own pace is a lot of fun! I do kind of wish that the Gyms scaled with your badges, but it doesn't bother me that much. I like a lot of the new Pokémon, Tinkaton is by far the standout and is rapidly becoming one of my all-time favorites. I thought the characters had a lot of personality and were written well, and I found the story to be pretty good. Arven and Penny's stories especially were very effective, I was emotionally invested in helping them out, which is more than I can say for most Pokémon stories, especially compared to the bland and forgettable characters and story of SwSh. Probably my third favorite overall in terms of story, behind BW and SuMo. The music was fantastic, particularly the Team Star battle theme and the Area Zero music. Terastalizing is my new favorite of the 3D Generational "gimmick", since it's a bit more strategic (and much less pandering) than Megas or Dynamaxing. Overall, I'd say it's a massive step up from Sword and Shield in basically every way, though I cannot comment on its improvements from Legends Arceus as I skipped that one (which from what I've seen and heard, was a mistake on my part).

But on the other hand, by god this is the single most unpolished Pokémon game. The frame rate is incredibly inconsistent, to the point where I was worried the game had crashed because it had dropped to single digits. The draw distance is laughably small, which when paired with some incredibly small Pokémon models leads to numerous unwanted encounters with Pokémon I couldn't even see, in addition to shadows manifesting five feet in front of me. There's a lot of Texture Warping, something I was under the impression died off during the PS1 days but apparently not. Pokémon can spawn in walls (and clip through them to start encounters with you). Shadows disappear and reappear depending on the camera angle. And most annoyingly of all, the game can crash. Now I'm lucky, I only had the game crash around 4 or 5 times, but that's still unacceptable. I've never had a Pokémon game crash on me before, out of the thousands of hours I've poured into this series. It's disappointing to see it like this.

I will again stress that the game is a lot of fun, and I sincerely hope patches fix it more so I can rate it higher. I really did love this game, it basically rekindled my love for mainline Pokémon that had been all but killed off with the sheer mediocrity of everything Post-B2W2, especially XY and SwSh.

Mixed on this one, as a longtime fan of the games. Ordinarily, I'd write a review, but I think a simple pro/con list might be better suited for this.
Note: I am referring to the package as a whole, including the DLC. I had not played Origins until Plus was released.

Pros:
-all non-game gear games are in widescreen
-mission mode pretty fun, if simplistic
-cute new cutscenes
-background islands are cute
-playable amy rose
-coin system feels like a good replacement for lives, and makes the special stages bearable (I am a longtime special stage hater)
-probably the best official way to play the classic Sonic Games
-metal sonic is in this game

Cons:
-Sonic 3 has several music tracks replaced with terrible substitutes, meaning no Ice Cap Zone jams
-you apparently have to pay extra to look at the islands(???)
-you have to pay extra to play as Knuckles in CD for some reason (?????)
-new cutscenes are so short and don't add much to the story at all, and some from the trailers are not in the final game
-"classic mode" is not the original rom, instead just the "anniversary mode" but cropped
-bizarre lack of other Genesis Sonic games, namely 3D Blast, Mean Bean Machine and Spinball, despite the Spinball Island being featured and the Plus dlc including the Game Gear version of Mean Bean
-strange omission of far superior Master System versions of the 8-Bit games, namely Sonic 1 and 2
-how has Knuckles' Chaotix STILL not been rereleased to this day
-Amy is fun, but underwhelming in comparison to her moveset from Advance 1
-music player missing the Past versions of the CD tracks for some reason (???)
-absolutely terrible sound quality in the Game Gear emulation, getting the speed shoes and invincibility in GG Sonic 1 resulted in a horrible cacophony of noise
-Sonic CD's menus flip between the classic menu and the 2011 remaster menu with every selection
-physics are not exact to originals, but seem to be better now than they used to, resulting in a lot of sequences not working as intended (still fairly close, but there's no excuse for it to not be 1:1)
-really underwhelming selection of concept art, especially in the physical art book
-sadly the best official way to play the classic Sonic games
-no playable metal sonic

I still enjoyed my time with it, if only because Sonic 1, CD, 2 and 3&K are all excellent games. I suppose I would recommend it to first-timers, since it's the best official way to play these games, but it's disappointing knowing how much better it could have been. An entire generation of people are going to experience Ice Cap with the wrong music.
As a collection, it's inferior to Jam, Mega Collection Plus and Gems Collection. Game Gear Emulation was done better there, too.

tl;dr: It's fine, I'm just annoyed about what could have been and the lack of effort and blatant greed. Play it on PC with a mod to restore the S3&K music.
or just play S3 AIR