It's the most interesting Sonic game, and not everyone knows about it.
Sonic CD follows the usual 2D Sonic gameplay, with Dr Robotnik as the main bad guy and Sonic stopping him. However, Sonic's friend Amy gets kidnapped by Metal Sonic, and Sonic must save here from him and foil Robotnik's evil plans once again.
How does he do this? When Sonic hits a time post and reaches a certain speed, he can go back to the past and stop Robotnik's robots from appearing in the future. However, sometimes Sonic will go into the future, and discover a horrible world ruled by Robotnik, or it could be a peaceful world. It all depends on what Sonic does in the past. It sounds a bit complicated, but to be honest the idea is pulled off amazingly well on Sonic CD.
All of the old Sonic moves are here, such as the Spin Dash and the jumping. All of the standard type of levels are here, such as a Green Hill type level, an underwater level, a Robotnik base level, a speedy level, and a bumper-themed level. All these levels are very interesting, and there's seven in total. Most of these levels are quite long, and do have some challenge in them, especially the bosses. The special stages are set in a 3D type level, which involves you having to destroy UFOs in a certain time. There are plenty of hazards, and it gets rather difficult. When you complete a special stage, you get a time stone instead of a chaos emerald. These time stones determine what ending you'll get at the end of the game.
There's also a save feature on Sonic CD as well. This is very useful because it keeps track of your progress in the game, and once you complete the game you can go back to any level you want. There's also a time attack mode and a sound test, plus a debug mode which is very fun and useful to use (I won't tell you how you unlock it though).
Now for the biggest highlights of Sonic CD. The music is absolutely amazing in this game! It has the best Sonic soundtrack ever and all of the tunes in this game are great! From the Sonic Boom to the Stardust Speedway tune, there's no music here which you won't like. All the classic Sonic sounds are here as well. For once, the sound steals the show in a Sonic game!.
The graphics are stunning as well, especially for a Sonic game. The amount of detail that's used in the zone backgrounds is amazing, plus all of the highly detailed enemies in the game. Sonic looks a bit basic but the rest of the game is highly sharp in graphics. You'll need at least a mid-ranged PC to play this game at a very smooth framerate.
Unfortuantly, Sonic CD doesn't seem to work on Windows XP, but wait! There's a patch that exists on the internet which fixes this problem out (well, not really, it makes Windows XP think it's a Windows 95 operating system to run the game). However, I can't seem to be able to run the game in fullscreen and it keeps crashing whenever I try it. It might be just a problem I have though.
So overall, Sonic CD is one of the best Sonic games ever made. However, it's not easy to get hold off on the PC anymore, so your best bet is to buy Sonic Gems Collection to play it. Hell, get Sonic Gems Collection just for this game, because that's all you'll need it for! That Gamespot review is a joke, because this is one of the most underrated games in Sonic's history.