Sonic CD is a pretty good game that took advantage of some great ideas, but after multiple playthroughs, I felt that it was marred by noticeable flaws which dragged down what would otherwise have been a solid game.
Time travel enhances the exploration factor to Sonic's already multi-tiered level design by creating secondary objectives that require seeking out specific objects, in addition to changing the terrain for each time zone. This gameplay element is reinforced thematically; if you don't find the badnik spawners, the zone enters a Bad Future where Eggman has soiled the landscape... but if you do destroy them, things look even better than in the Present, and as a bonus all of the enemies are destroyed. I always feel a certain sense of satisfaction from going to the Past, destroying the spawner, and then going into the Future of that same level to enjoy the scenery and rack up rings and 1ups ("I did this, and now I'm reaping the rewards"). You can also create a good future by collecting all of the time stones (which also gives you the "good" ending, as miniscule a difference as it is).
There are two main problems with this factor of the game: the sole method of travelling through time is obnoxious, and the secondary objective is ultimately not well incentivized.
Time posts allow you to try to build up speed; once this hedgehog reaches 88 mph, a cutscene will play and Sonic will then find himself in the same spot forward or backward in time. Well... it's inaccurate to say you go through time by "building up speed", it's more that you go at "run speed" for a certain amount of time, then the HUD indicator starts blinking, and if you're still at running speed for a certain amount of time, you complete the time travel sequence... otherwise you lose the ability.
This system is stiff and unforgiving. Sonic's camera doesn't give you enough room to plan ahead for certain terrain when going fast, which makes it very easy to lose momentum just after you reach the "blink" mode. Unless you stop, walk around the act, and strategize a track that you can sprint through, it's usually more favorable to find two springs close to each other so you can bounce around and circumvent the challenge in a stupidly easy fashion. It all feels very clunky and not exceptionally well skill indexed.
It would have been more favorable if Sonic could go through time by finding giant rings or something similar, like at the end of the levels or in Sonic 3's hidden rooms. Actually, why not make it so that Sonic's peel-out mode instantly transports Sonic to the next time zone so you don't even have to create this ugly speed system for it? The peel-out isn't even that great of an addition to begin with; the spindash lets you do pretty much the same thing but in roll mode.
The other thing is that it's honestly a lot easier to create good futures by just beating the special stages. You don't have to go through the time post ♥♥♥♥♥♥♥♥ or hunt for the spawners, and plus you only have to do it seven times if you're good enough (there are twice as many spawners in comparison, not counting the Metal Sonic holograms), and so if you just want to beat the game optimally and quickly, you really just have to ignore the Past for the most part, which devalues a good chunk of the gameplay. And hell, even if the game allowed you to start every level in the Past, you'd still have to figure out where the spawner actually is, which sometimes feels like finding a needle in a haystack. And once you find it, there's usually very little opposition keeping you from actually destroying it, which makes it a bit underwhelming.
I love the idea behind the time travel gimmick, and it's a great fit for Sonic's levels because of how diverse and multilinear they are to begin with. However, I also believe it's not as good of a fit for Sonic's _mechanics_ as it would have been for other platformers, like Metroid or Castlevania, or even something fairly simple like Mario. Levels with time travel dynamics work best when it creates design space for puzzles, exploration, and combat. While Sonic does inherently have a good deal of exploration, it's far more focused on the momentum-based physics than it is on gimmick puzzles or even badniks, and so it's difficult to really expand on this through the game's gimmick.
Speaking of badniks, this game is hands-down the easiest of the classics. Sonic Team had some of the best ideas for bosses in this game, but they're squandered due to positioning/blink time exploits on the player's part as well as overall just being too forgiving. There are very few death pits in this game in general, the only one even coming to mind being on the last level, and the final boss itself is a joke. It's still possible to game over if you're new at the game, but otherwise the difficulty curve is very shallow.
The environments are really gorgeous and diverse, probably some of my favorite from the 16-bit era. IMO this is what actually makes the game memorable. I will say that the backgrounds are a bit too "busy" in terms of contrast, which sometimes makes it a little difficult to follow the action.
This port allows you to choose between the American and Japanese soundtrack, and both are nice in their own ways. The Japanese version has the kind of "bounce" and focus on melodic motifs that you would expect from the Sonic franchise, but Spencer Nelson centralizes more around rock and ambience, which is a great fit for the Future levels. This port in particular creates looppoints to all of the original tunes, and some of the American tunes even take advantage of extended arrangements previously only featured in the album "Sonic the Hedgehog BOOM". Overall a very nice sound package. (Now if only we had American "Past" variants... )
If you already knew about this game and are wondering which version is the best, it's definitely Steam's. If you're wondering if this is the go-to Sonic game... some diehards will tell you otherwise, but I would say Sonic 2 and Sonic 3 & Knuckles are preferable in terms of gameplay. The boons of the time travel gimmick aren't present in these games, but both games offer a more satistfying challenge and are quite a bit longer than Sonic CD. If you just want a decent platformer on Steam, then I would recommend this.