Ah, Sonic CD. Arguably one of the only games worth owning on the ill-fated Sega CD, this title takes the classic Sonic formula of high-speed, momentum-based platforming and adds time travel to the mix, and this is the feature that will make or break the game for you. Scattered throughout the levels are signposts with 'past' and 'future' written on them. Running past one of the signs, then building up enough speed, will send Sonic backwards or forwards in time. Each level actualy has four variations: the past, the present, and the good and bad futures. This generaly works well (although I did get once stuck in a wall after warping to the past), with the differences in layout between the time periods providing for some interesting platforming, and adds to the game's greater focus on exploration.
In each Zone (called Acts in other Sonic games) there are two objects to destroy in the past: Eggman's robot generators, and Metal Sonic holigram projectors. Destroying the projectors results in animals roaming about the Zone, to no effect whatsoever. However, destroying a generator will result in all enemies in the Zone instantly being destroyed, as well as changing the future. You see, if you travel to the future normaly, it's a corupted wastland, a bad future. By destroying the generators, you can alter the future, creating the harmonious good futures instead. However, this only applies to the first and second Zones of each Round (called Zones in other Sonic games). In the 3rd Zone of each Round you start in the future; if you destroyed the generator in both other Zones, this will be a good future, and if not, a bad future. Locating the generators can be quite tricky, due to the non-linear, multi-level stage design, and the Zones in this game can be hard to navigate. In addition to all that, to get the game's true ending, you have to make good futures in every single Zone. Or, instead, you could get 50 rings during a Zone, reach the signpost, and enter 3D, UFO-Smashing special stage to collect the Time Stones. Getting all 7 will automaticly get you a good future in every zone, and and guarantee you the good ending.
This is where the game's main problem rears it's ugly head: It claims to have a focus on exploration, but there's really no point to actualy exploring. Getting the Time Stones is so much easier then smashing every single generator, and once you have them, the level design can seem like more of a hinderence then anything. Worse, if you don't care about geting the good ending, and just want to play this like any other clasic sonic game, you might find the level design to be an outright hindrance, with few places to pick up and keep speed. Whether you find the level design to be a blessing (huge, beautiful stages, full of interesting gimmicks) or a curse (massive, mazelike, slow hells) will depend mostly on your playstyle, and is almost entirely subjective.
On a more positive note, this game looks realy good. The game's been updated with 16:9 widescreen support, and you can use one of various filters if you wish to. The game also sounds great, with both the US and JP/EU soundtracks being available to choose from. Each version is great, and having them both is absolutely fantastic. As this is an updated rerelease of the game, various improvements have been made, including tweeked collision detection in both the main game and the special stages, the above-mentioned dual soundtracks and widescreen support, and Tails as a playable character, unlocked after beating the game with Sonic.
Although level design may be subpar for some, Sonic CD is still a good game with a lot to offer, and if you enjoy hunting for secrets, the game has you covered with large levels, each possessing four variants, and lots of little touches that fun to find. Having exploration as it's main focus, rather than being secondary to speedy platforming, may make it the black sheep of the Genesis titles, but the game still nails the smooth platforming that defines classic Sonic. This version is without a doubt the best way to experience Sonic CD, and for just $5, I'd definitely reccomend this game.