HUGE Sonic fan, btw. Beware: long message
FAI, Sonic CD is THE BEST SONIC GAME OF ALL TIME. Period.
If you are late to the game, do yourselves a favor and play it. It STILL serves as the best example of a fun, "pure Sonic" (w/out any other friends) game 15 years later.
This game blew my mind wide open when I was young. My parents would be thankful (PSYCH), but Sonic CD is not only the reason I'm hooked on video games, but also why I am an artist and a wannabe musician too. Yeah, I'm dead serious when I say this game literally changed my life. Well, anyway, back to the subject.....
The game mechanics are what make Sonic CD really shine. The time mechanic to me was a stroke of genius that added a huge layer of depth to the game -- it gave you a fun reason to play it over and over, rather than just "if I feel like it". For example to those that don't know, in this game you could time-travel. Each stage was just like any other old-school sonic stage, except they were littered with spinning signs (like the ones you pass to complete the level) that say "Past", "Present", or "Future". It's been a while since I played it, but if you pass a sign and travel a full speed for a certain amount of seconds, you disappear in blinding flash of light and travel to whatever time listed on said sign (kinda like the Mandalorian in Back to the Future. I think that's the car's name).
Each "time period" of a certain level was basically a remix of the original, with new music and redone colors and background, and sometimes, new enemies as well. Each stage had 4 different versions (past,present,good and bad futures), to which you could technically say it was the biggest standalone old-school Sonic game ever (without counting linking two games together (like Sonic 3 & Knux).
I loved it, because it gave the game an air of mystery and wonder, like I couldn't wait to see the next time period and see how much it changed from the present, especially the good future. It was fun, too, because the levels themselves changed to where certain areas and abilities can only be reached by going to a certain timeframe, and the good future could only be reached by destroying a certain amount of Robotnik's machines located throughout various points of the "past" stage version.
And this is just one part of the game. I also personally believe the stage design in this game is alot more fun and geared towards speed than any other old-school sonic game. Sonic 2, for example, as much as I loved it, always pissed me off because of the fact that I couldn't go fast for too long without running into a wall and coming to a complete stop. Its a trait that all old-school sonic games share, but comes much less often in Sonic CD than other games, giving you a true sense of speed.
I'm not even going to get into the other parts of the game, I could write (literally) for hours...like the graphics (beautiful, just check out Stardust Speedway when you play the game), cool bosses (best example, the original Metal Sonic race), or MUSIC (top 10 game soundtrack of all time, and I'm talking BOTH versions).
Whoever mentions Sonic CD really unleashed a beast w/in me.
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Oh, and btw, Sonic Unleashed, while not perfect, is alot of fun, even with the werehog parts! I should know because I actually BEAT THE GAME (to any naysayers who haven't even really picked it up long enough to criticize it). The werehog parts were done well enough to keep players interested -- while the platforming bits were frustrating at times and displayed the worse parts of the game (for example, certain areas of a stage with an unnatural camera view would screw up controls and cause an unfair death), it was still ok, and the fighting parts were alot of fun, b/c of all of the combos and other cool abilities Sonic Team gave him. To me, they should have had Knuckles (who never gets any Sonic Team love) instead of Werehog, and it would have been much better received, and maybe even praised (it basically would be Sonic and Knux 2).
The daytime bits were pretty much Sonic perfection -- this is what I have been waiting for since Sonic Adventure 2 first came out. They really give you a sense of speed (I mean, you literally move through the screen faster than in most racing games!) and did a great job on it.
At the end of it all, Sonic Unleashed is basically Sonic Adventure (1) with more and better Sonic (daytime) bits, and less friends. If you like SA1, then there is no reason you shouldn't like SU just as much or more.