Sonic (the Hedgehog) CD was the first, and only (DAMMIT) Sonic game on the Mega/Sega CD system. At first glance, the game is well presented, and has many features common to its cartridge predecessors (although it is much more like Sonic 1 than Sonic 2), such as the speed and graphics. The animated introduction makes use of the FMV capabilities of the Mega/Sega CD (which sucked more than 1000 episodes of Dawson's Creek, and Sega know this), and the music/samples are great.

Ok... first things second (EH?). This game looks AWESOME. The Mega CD version plays at a NICE 30 frames a second, making Sonic's speed show just as much as it did in the previous outings. The PC version (which Sega don't sell anymore... DOH) can run at 60 frames a second... that's the same speed as Sonic Adventure 2!

The sprites resemble those of Sonic 1, which isn't a bad thing (Tails wasn't in Sonic 1). Some new animations have been added, such as the 3D twisty thing in the beginning (couldn't get a screenshot of that...). All of the backgrounds look erm... backgroundy? Ok, they look just like Sonic 1 backgrounds. The 3D special stages and the animated intro screen save the graphics, as they look great! Although there is new stuff here, it's not enough for anyone to go mental over.

This game plays SO well. Like the other Sonic games, the average person could put this game into the required console and start playing. Providing the average person could figure out how to put the game into the console that is.
Each level is split into 3 acts, as usual. However... each act has a present, a past, and 2 possible futures... created upon how you screw up history. To create the either one requires using the PAST and FUTURE posts and speeding in one direction. Finding BOTH the Metal Sonic hologram and this robot-making machine, and effectively destroying them creates a good future full of fluffy animals. Not bothering to destroy them (LAZY!!!) will create a future that looks like L.A. With all the Zones are split so many different time zones, decided upon by the players actions, and the sheer size of the levels, the game is HUGE!

Sonic CD has many nice features and graphical touches. The size of the game is obviously a great feature, but Sega finally figured out that people like to race Sonic and their skills against the clock. So they shoved a nice Time Trial into the options. Any levels that have been completed can be completed in the quickest time possible. Simple as that Simon dude... with the pies and all that.
Other small things that probably don't count are the Sound Test (given in the PC version by completing the game), and access to watching all the animations in the game (the opening and ending screens!). And I think it's worth mentioning that Tails isn't in Sonic CD, but he's replaced by the equally annoying Amy Rose.

Having said the Mega CD had NASTY FMV, it did have one thing that the Mega Drive (or Genesis for my fellow Americans) couldn't manage... ever... decent quality sound! The sound for the English version is GODLIKE compared to other 16-bit consoles at the time. As this is being written, my PC's sound card is working overtime to play the background music for Stardust Speedway's bad future (RealPlayer is needed for the sample).
The usual Sonic jumps and spring sounds are in there, (OH NO... the Chaos emerald music has changed!!!), and Sonic actually does talk! Collect an extra life and he shouts "YEA", leave him alone for a few minutes, Sonic gets annoyed, shouts "I'm outta here" and jumps offscreen, thus proving that he is very annoyed... or something.

This game may be stunning in most departments, but, much like solar power, it does have its faults. The game is HUGE, but it feels like something more could have been added, such as a 2 player option (with Sonic Vs Metal Sonic!!!), or difficulty settings, or even more levels. The game doesn't feel unfinished, but more could have been done (although saying that, most Sonic fans would give a lung to have this game).
Having Super Sonic in Sonic CD would have been nice, but seeing as the Sonic collects "Time Stones" and not Chaos Emeralds, that would be an impossibility (due to Sega making the plot a lot harder to follow... as if it wasn't hard 'nuff).

At the end of the day, a game can boil down to 2 things, gameplay and life-span, of which this game has heaps! The PC version is a monster compared to the Mega CD version, with an instant level select upon level completion, and the theatre/sound test not appearing until the "hidden" time attack scores are beaten. The original is still better, as it does not freeze as often... (I hate PC games for that very reason), but now, you'll be spending HUGE amounts of time and cash to get it. But if you must own a Sonic game, make sure it's this one!

Sonic CD: The Scores on the, uh, Doors (lucky I'm not getting paid for this!)

Graphics: 9 - A great way a 2D platformer should be done. The FMV sequences are done very well! Definitely better than Sonic 2!

Sound: 9 - CD quality sound here... with nice effects. Would have been 10 if Sega kept the Chaos emerald sounds. "YEA!"

Originality: 5 - The 3D section and Metal Sonic saves this score, but Sega stuck to the "If it ain't broke" formula which made Sonic at the time.

Life-span: 8 - Wanting to see all of the time zones and the proper ending takes a LONG time.

Final Percentage - 97% A fine example of how a game is made. Shame about the features mind...