Updated January the 2nd, 2023
The key to playing this game and getting good futures in later zones is to only collect the 7 Time stones then every zone will have a good future with less enemies. I've bumped the score from a 5 to a 6 to reflect my changed feelings as not bothering with time travelling adds to the fun. Now I consider the game a fair game to play especially at a cheap price but certainly not at what the Sega CD cost when first released for that game to ever be played.
Original
I am a long-time fan of Sonic the Hedgehog 2 way back when I was a kid in the 90's. Didn't own a copy for myself and my sister but we rented it a bunch of times from the video store. It really was the first Sonic game we played and because of that we disliked the first as it did not have the spin dash that was introduced in the sequel. As such we only rented it once which looking back that's not a good excuse, but kids can be particular to such simple things. However, I sure am glad either of us didn't have the Sega CD expansion as this game doesn't know the meaning of the word "speed" that both my sister and I loved about Sonic to begin with. Or "fun" for that matter.
My hugest gripe I have with this game is all the level designs. They are all assembled haphazardly with no consideration for players who just want to speed to the end without hitting every obstacle along the way. There is no flow to speed and continue doing so because there is a new mechanic introduced in Sonic CD which is time travel. Throughout the levels there are sign posts labelled past or future and all that is needed is to collect it and find a spot to run at a good speed until sparkles appear and then keep the momentum going for 5 seconds without slowing down. Sounds easy right? Wrong!!! Remember what I said about the levels having no flow and obstacles all over the place. For a character about speed the designers of this game must have thought they were making a game for another mascot.
This game is focused on exploration more than how quickly Sonic can run from start to finish. The only reason I can think of as to why height has played an important role was because if the player can travel to the past, hidden throughout what I can best describe as a maze structure is a machine that creates Badniks (the robot enemies) and if you destroy that machine a good future is created as opposed to the default bad future. After destroying the machine, the player can choose to travel to the present and then to the future which will be filled with colours and animals with no Badniks. If both acts of a zone have good futures, then Robotnik can be fought in the good future.
Now all this time travel is completely optional but that really defeats the point of how the levels have been built to accommodate exploration rather than how fast a kid can beat the level without feeling lost. The early levels aren't no bad for building up speed but shortly around the fourth zone the choices of where to run become slim. I suggest ignoring the time travel and try to focus on colleting 50 rings and taking that to the end where a big ring awaits you to jump through. This will take you to the special stage where you are tasked with jumping at 6 UFOs to collect a Time Stone and bag a whole lot of points that helps to increase lives.
The issues I have with the special stage is that depth perception becomes a real problem as often times the player is either jumping too short or too high at a UFO and turning around is a real hassle. Sonic turns in wide births so the player could potentially be running around the UFO in a circle. There is a time limit of 80 seconds which seems fair but touch the water and that clock drains fast. If one is adamant to collect all 7 Time Stones, then I suggest quitting to the main menu before time runs out as the auto saving will restart at the beginning of the special stage. Prepare for a lot of restarting because those UFOs can be wily making it hard to get all 6 UFOs before the timer runs out.
As for how the game looks well it certainly isn't the game's selling point. Hard for Sega to justify the Sega CD expansion when your flagship game looks no better than Sonic the Hedgehog 2 from the year before. Actually, I would say it looks worst as the levels look uninspired artistically as if the whole game was built with a level editor than built from the ground up by graphic designers. Everything feels a little claustrophobic and too busy that I'm mostly looking at a maze fit for Spelunky which arguably looks better too. As for the sound well it's okay but nothing I'd want to hum to. Part of the joy of playing Sonic the Hedgehog 2 was listening to the music of each zone and humming while playing. Sonic CD has two soundtracks which are the US and Japan for the player to choose at the main menu, but I just can't get into this music from either as they feels dated and of their time.
If I was a kid who played this because my parents somehow lost their minds and bought my sister and I a Sega CD, then I would truly pity my parents for wasting their money as I highly doubt either of us kids would have enjoyed this faux pas of a sequel to the great Sonic the Hedgehog 2. The stores back in the 90's in Australia were pretty generous with refunds so perhaps if we were quick enough mom and dad could have gotten us kids an N64! We were late in getting a Sega Mega Drive.