Matched only by Sonic 2
Sonic CD is truly a good enough game to warrant the purchase of a SegaCD. The vibrant graphics, the stellar Japanese soundtrack, and the innovative time-travel mechanic all combine to make this my favorite Sonic game ever.
The game is split into 6 zones, each made up of the traditional two acts, and beginning with the ubiquitous "green" zone. The level design is excellent, and this contains some the fastest levels you'll see this side of Sonic Advance 2.
But it's not just a speed-fest. This time around, the Chaos emeralds are replaced by Time Stones, and it's not just a matter of collecting 50 rings anymore. To fully save each zone, Sonic must use his speed to literally warp to the past and future of each zone, in order to destroy Dr. Robotnik's and Metal Sonic's evil history altering machines. This means each act has a "past" and a "future", each with a slightly different layout. Of course, like all Sonic games, you don't HAVE to free all the zones and collect all the Time Stones to beat the game, but you do to truly "complete" the game.
The negatives..well, 6 zones is a little too few, in my opinion. But the past/future mechanic pads these quite well. My other minor niggle involves the slightly non-traditional spin dash mechanic. This game was developed in parallel with Sonic 2, and both games implement it differently. Sonic 2's version was the one adopted by future games, making this one feel like the odd one out. But you get used to it.
Also, be warned that this game has two soundtracks. The Japanese and European releases had the original, and best, soundtrack. For some reason, Sega decided to totally replace the soundtrack for the US version, with a decent but inferior soundtrack. The original Japanese/EU soundtrack is the best Sonic music I've heard, and fits the game much better. For this reason, if at all possible, get the JP/EU SegaCD version, and stay away from the US SegaCD and PC port versions.
The Bottom Line
This game is matched only by Sonic 2, and that should be enough to make you want to play it.