Played: 2011 Christian Whitehead rerelease. I'll review the port itself later. This is just for the game itself. I technically did play it on this platform though

I like this game. I like this game? I think so. Maybe. But before I get to that, first, the stage ratings.
Palmtree Panic: Inoffensive, good GH-like stage. 4/5.
Collision Chaos Zone: This stage normally is fine to play but the Past version of it is absolutely GORGEOUS. This is the stage I couldn't beat as a kid, I got to the boss fight but I was a dumb kid 3.5/5.
Tidal Tempest Zone: Out of this game, Sonic 1, and Sonic 2, this is the best water level. Ain't perfect, but I enjoy it a fair amount. 3/5.
Quartz Quadrant Zone: It's alright. On the replay, I died 4 times on the boss for some reason. 2.5/5
Wacky Workbench Zone: You could find some of it in the last zone, but this is truly where the Sonic 1 bullshit starts to kick in, except this time, it's also very confusing. WHERE AM I GOING 1.5/5
Stardust Speedway Zone: Same as last zone, where am I going? Music's good, and the boss fight is pretty interesting. Unfortunately, it's still a 2/5.
Metallic Madness Zone: My thoughts.. 0.5/5.
Well, it's not looking too good for Sonic CD right now. That's about a 2.5/5, rounded up. But I'm not done. Sonic CD is much bigger than just the stages. For example, the intro cutscene. Whoa, there's an intro cutscene? I wonder what it's li-HOOOOOOOLY SHIIIIIIIIIIIIIIIIIIIIIIIIT
The intro cutscene is SO GOOD. I don't know what song it uses in the Japanese version, but I don't care. Because Sonic Boom (the SONG) is PEAAAAK. Intro cutscene is peak. End of discussion. Speaking of the music, the whole OST is peak. Both the JP and US soundtracks. I definitely prefer the JP soundtrack, but from what I've heard of it, the US one is pretty good. The JP soundtrack just sounds more Sonic to me. I think I might honestly like it more than Sonic 2's soundtrack, but that'll probably switch on the daily. As AAAAAAASSSSSS as the stage is, Metallic Madness (Present) from the JP OST is probably the best song, in my opinion, at least.
Now, to the stinky part. First, and definitely most, the level design. This is probably the thing that most people rag on the game about. And I...yeah, I agree completely. The level designs start inoffensive until about the halfway point, until they start decreasing and decreasing from then until it reaches Metallic Madness. Talk about a headache. I genuinely had to look up a map of the zone just to get through. IN A PLATFORMER. PLATFORMERS ARE SUPPOSED TO BE THE EASIEST GENRE TO GET INTO AND UNDERSTAND!! This problem also affects probably the most interesting new gimmick, that being the time travel. Yeah, weren't expecting time travel from a game called...Sonic CD, were ya? By passing a sign in placed in the levels, if you go fast for a while, you then travel either to the Past or the Future. If you destroy a robot-making machine or whatever in the Past, you will get the Good Future of that zone. If you get the Good Future for every stage in every zone of the game.....you get a DIFFERENT CUTSCENE FOR THE ENDING. WAHOO. Plus, it's difficult to time travel because of the level design. You would think it would be super easy to go fast for a long time in a game with huge stages. But no, the stages are packed and sometimes mazelike. It's very difficult, and when you're not trying to specifically time travel, sometimes you can accidentally do so by just randomly going fast. And even when you're trying, sometimes you can accidentally go past the opposite sign, so you can't go to the time period you want to. It's just plain TEDIOUS.
So, yeah, that's it, that's the end of my rant.
This game is not an underrated gem.

Gonna try not to be biased as this one of the games I played for hours on end through the years until now as well!
I speedrun this, so I'm always happy for a buggy glitchy game, But the glitches in this are beyond me.

Achievements did not appear for the game on release week, one is currently unobtainable, and its mostly just grinding. How fun, lots of missed opportunities for some creative ones.
The loading sound was removed and changed for the 1st game, everything else worked fine on that, Now the second game, that's where the problems come in.

Missing assets on Naboo, like an entire wall is see through, Enemies and allies Even myself would spawn out of bounds on Naboo.
The new hero hunt mode for all maps completely breaks AI, on Kamino every enemy had no idea where to go, so they resorted to spinning in a circle then running off the edge.

Galactic conquest is broken if you play as the Empire and try to buy a new upgrade I was greeted with a crash every time, meaning I could not play that mode unless I was CIS, Rebels or Clones :(

Apart from that, the game is basically the same (if its not broke why fix it) In this case it was the company trying to add new things like the mode that broke it :(

Wait for Sale on this, or just buy the old ones on the xbox store. Much better.