Sonic CD is the perfect example of looking at a flawed product (Sonic 1) and making most of the same mistakes over again, but also somehow making it way cooler.
Where Sonic 2 actually feel like a sequel, CD feels closer to an expanded version of the original. The level design, while an improvement in some ways, still suffers from sections of slow platforming and confusing layouts, along with having maybe one of the worst levels of any 2D Sonic game to exist (Wacky Workbench).
The time travel gimmick, the main thematic focus of the game, ultimately doesn't amount to much. The entire game can be beaten, with the good ending, without ever travelling to either past or future. More often than not you'll be frustrated at the time travel animation breaking your flow, and potentially leaving you in a part of the level with a changed layout you weren't expecting. The first time it's neat to see the entire game shift color palette and layout, but by the end you'll be avoiding those time posts like the plague.
The special stages are also an..... acquired taste. A mode-7 like effect but with multiple moving objects that you need to hunt down and destroy, it can be difficult to gauge just how fast you're going and what the actual depth of objects are on the field.
But despite all this, the vibes. The vibes! Sonic CD oozes personality in a way none of the other 2D titles do. Visually the sprite work is a huge jump over Sonic 1, with the overall art direction and the addition of the opening and ending FMVs really giving the game a 90s anime vibe. The CD-quality music is excellent, with both the US and JP soundtracks having their own strengths, and acts as one of the few incentives to actually time travel in order to hear the remixed versions of each song.
The introduction of Amy Rose and Metal Sonic to the overall Sonic roster are two fine additions, with Metal Sonic being a standout design that has remained mostly unchanged to the present day. The stage involving a race with him feels like the game finally trying to make good on the idea that Sonic is supposed to be fast.
Sonic CD is a game that feels like it didn't learn all of the lessons it needed to after Sonic 1, but fixed just enough problems that the personality it exudes helps elevate it above the original.