Sonic CD has been released for Xbox Live and PSN for 5 bucks and mobile devices for 2 bucks. That's a no brainer if you ask me. But if you want more details keep reading.
The first thing you should know is that this re-release of Sonic CD is more than a throw together port like the other Genesis/Mega Drive releases for XBL/PSN. This version was rebuilt using the Retro-Engine developed by Christain Whitehead, otherwise known to the Sonic community as "Taxman." Everything has been retooled for modern gaming systems, including wide screen support, remastered audio files, constant smooth frame rates, updated physics and control options.
Not too much has changed from the original version of the game, except you'll notice that Sonic controls closer to his Sonic 2 counterpart. The spin dash from Sonic 2 and 3 has also been added which allows you to rev up by repeatedly pressing the button. For the purists out there you still have the option of playing with the original Spin Dash from Sonic CD as well.
Another welcome addition is the ability to play as Tails after finishing the game, which was not featured in the original Sonic CD. He comes complete with his flight ability as well as swimming in Tidal Tempest. Some tweaking of the levels was done to compensate for his ability to fly, which was mainly just fixing the stages so they would scroll higher.
As for the overall gameplay package, your enjoyment may depend on when exactly you got into Sonic in the first place. I've read and heard several comments on how the stages are much more difficult and not like traditional "Sonic stages." This is true in some sense as Sonic CD is more platform based and has few instances of long distance free running segments. The game is still about the speed, but you have to be more creative and possess more reflexes than simply holding right, which may turn off younger gamers or those newer to the classic series. I should also note that the main idea of the game is the time traveling mechanic which is meant to be used often if you're going for a good ending. This expands each act by adding a past, present and future layout for each. For a good time I recommend a thorough exploration of each layout before trying to clock those speedrun worthy times.
My favorite thing about this version is widescreen support, which allows you to see more obstacles on the way and alternate routes above and below. This made the Stardust Speedway race with Metal Sonic much easier than in the original. Not to mention finding those pesky robot generators and Metal Sonic Holograms in the past.
Graphically you are presented with three different options and they all look fantastic in HD. The smooth option is more than just a blurry effect that they usually apply to classic re-releases. Everything looks sharp and still has a hand drawn look. For the purists out there you're going to want to stick to the default or nostalgia filter which displays each raw pixel of the game. The game now runs at a constant 60 frames per second from what I can tell. The anime opening and ending cutscenes are included and remastered for the best presentation outside of Sonic Gems collection.
The improved framerate may either help or hurt your performance in the special stages. Since I learned these stages on the PC version of Sonic CD back in the day I had to get used to the new framerate and relearn the timing of my jumps because the special stages didn't feel as fast or as twitchy as before.
All of the audio files have been remastered from the original sources. Some tracks have even been sampled from the updated ones from the Sonic Boom album. Or if that's not your flavor you have the option of playing with the original Japanese soundtrack, all from the start with no unlocking. The tracks have also been reworked to loop properly instead of that awkward silence while the CD reloads the song which was typical of CD Rom games at the time.
It's safe to say that this is now the definitive version of Sonic CD. The small price tag and the fact that this isn't just another emulation warrants the purchase alone. Even if you're just into platformers you owe it to yourself to check this game out.
-Not just an emulation but a complete rebuild of the classic game, yet everything is intact from the original.
-Widescreen support and multiple graphic display options.
-Multiple control options. Controls are very responsive.
-Remastered audio files.
-Tails as a playable extra character.
-Level design might not appeal to players more accustomed to Sonic's more recent games.
Tips, tid-bits and possible spoilers:
-Know that achievements are disabled while you're playing as Tails.
-The unlockables in the extra section are all related to your time trial performance. Try for under 25 minutes for your main acts and under 5 for the special stages.
-The debug code as well as the extra images and easter egg codes are still intact after you unlock the sound test.
I suppose I could list these as cons for the game, but they don't really matter that much to the overall experience.
-Obviously since Tails wasn't meant to be included in Sonic CD, he has no ending. It would have been nice if he was given some sort of sprite based cutscene or something but again this doesn't matter that much.
-If you're playing with the Japanese sound track you'll probably notice that there aren't any vocals for the opening song, Sonic You Can Do Anything, otherwise known as "toot toot Sonic Warrior." This was due to licensing issues with the lyrics and the death of their owner.