The interesting thing is how the levels change according to how you play
Gameplay: While its usually nothing outside the Sonic norm, the Spin Dash was included, a definete good point, though Sonic accelerates at a good pace just by running on flat ground, there are areas where you may need to speed up quickly to get under falling platforms and such. Each level has a distinct different style, from the oceanic Tidal Tempest to the somewhat out of control bouncing of Wacky Workbench and the speedy twists and turns of Starlight Speedway. The only thing I don't like that much about the game is how it actually forces you to slow down; when Sonic obtains the ability to travel through time, he has to run at full speed for a full 10 seconds in order to do so, and if he ends up stopping abruptly after 6-9 seconds have passed, he loses the power, having to find another of shockingly few time posts. Having said that, there are some areas of levels specifically designed to allow Sonic to time travel quickly and easily, most common being vertical half-pipes with springs at the bottom, by spinning into the spring his vertical ascent carries him down the half pipe again, hitting the spring and retaining his speed. The special stages are inspired, but perhaps the least fun parts of the game, with the speed of Sonic's off-rails movement getting in the way of the player's ability to concentrate, the fact the UFOs are sprite based and increase in size jerkily also make it very hard to judge whether a jump will hit or not. Also the bosses, taking place in the future of each world, are very varied, with some bosses utilising very clever ideas. The water-based boss covers itself in a shield of bubbles, which Sonic needs for air, but cannot pass through. The essence of the boss is that the water becomes more dangerous as the fight progresses. Each boss uses an element of scenery in a level to good advantage, making it seem like the game's world is against you as much as helping you.
Graphics/Presentation: Amazing.The best part of the game hands down. As you play through the game, the game's world changes depending on what you do, for example if you go from the present to the future you see a distopian vision of the world you just left from, complete with an abundance of robots and factory backgrounds. If you return to the past from the present, you see parts of the level which haven't been built yet, and the backgrounds show the same with skies where there were metal walls etc. If you manage to locate a machine called a robot builder in the level (an object oddly easy to find, considering the size of the levels) and destroy it in a single attack, both the present and the future are affected, with robots which were unable to still exist due to the machine's destruction in the past blowing up automatically as Sonic passes their screen, falling down into brightly colored flowers when they hit the ground. Animals are also in abundance at this time, enjoying the natural beauty of the level's "Good Future." Destroying the machine actually creates a sense of the game's world thanking you for your efforts, it really brings out the feeling the game tries to portray. Also, though a smaller point in the larger scale, in a few areas the game's story is shown through a variety of in-game actions, similar to in Sonic 3 but much more subtle, such as near the start when Metal Sonic kidnaps Amy, Sonic is free to move around as normal (though Metal Sonic remains invulnerable throughout) also small voice clips such as a distinctive "Yes!" when Sonic gains an extra life are quite notable, and sound good.
Music: An interesting point, with no considerable upside or down. The game's intro was essentially the ground from which was built THE (in my opinion) BEST series of music tracks in ANY game series ever. It is short and fairly minimalist, but the rock guitar sounds loud and clear in both Euro/*** and American versions though the two different versions carry different styles of emotion (I prefer the U.S. intro, but that's as far as it goes with the U.S. songs) each version has its own set of lyrics too. The levels themselves have different music tracks for each time period, though they always retain the same melody, the key and pitch is altered (and in most cases entirely remixed) to give a feeling based on the time period. The music in the present could be considered your standard platformer fare if it wasn't for the high quality. The past music is very minimalist, sounding like music from the Megadrive in slightly higher quality, and giving a sense of loneliness. The music in the Bad Futures are always negative, with minor notes and machine-like noises prominent, while the Good Futures are very relaxing, chill out remixes. The music's style in general aside from the intro is very different to what Sonic gamers have come to know from more recent games in the series, having a series of techno-esque compositions utilising elements people would expect from the level's design. There is even the occasional lyric or three in the song, though very few unlike the Sonic Rush tunes, they are used to improve the music, instead of being used AS the music.
Conclusion: Sonic CD is definetly a game more to look at and listen to than it is to play, though dripping old school Sonic out of every megabyte put into its shining disc, so fans of the old style will definetly enjoy it. People who don't will enjoy altering the levels to their liking.