If you've ever owned a Genesis, odds are you've heard of Sonic. He was the thing that made Genesis so hugely popular, and he even managed to carry others like Master System and Game Gear well into the 90s. And of course, Sega wanted to extend Sonic to other systems of theirs like 32X and Sega CD.
Now, Sonic CD was originally not going to be Sonic CD. Back when the Sonic the Hedgehog 2 was in development, Sega had planned to have the same game on Genesis, Game Gear, Sega CD, and Master System. However, the four eventually wound up having almost completely different games. What Sega CD wound up with was Sonic CD, and it's considerably worse off for it.
Sonic CD plays much like any other Sonic game, meaning you run around and collect rings while trying to get to the goal. However, Sonic CD adds a time travel twist that makes things simaultaneously more interesting and more frustrating. If you hit a Past or Future sign and get going fast for long enough, you'll warp to that time period. The point of this is that when you're in the Past you can destroy a machine and make a good future. This is essential to getting a good ending.
Now, while Sonic CD gets the basic Sonic formula down, it makes several galling mistakes. It constantly seems like it's begging you to go fast, but whenever you actually get an opportunity to do this the game tgrows a roadblock in your way to stop you. In fact, Sonic CD just seems like an endless parade of bad level design. You have to practically memorize the levels just to get all the good futures and actually beat Robotnik once and for all.
And yet, You just can't help but like this game. It has an addictive quality that just begs you to come back for more, even though you know you'll probably hit OFF once you reach Wacky Workbench Zone (seriously, that zone SUCKS). But if you're hurting for a decent platformer for your Sega CD, this game will suffice. Just don't expect the next Sonic 2.