Sonic CD, in a word, is confusing. Like, holy crap, "how do you do anything in this game" confusing. There's a time travel element, but getting it to actually work takes a man like Doc Brown to get it to actually work.
If you've played a classic Sonic game, then you know what to expect: a nonlinear level design with an emphasis on going fast. This entry in the series definitely keeps up that tradition, but with the significant addition of the aforementioned time-travel mechanic. You can go through the whole game without using it, but if you want to get the good ending to the game, you have to follow a very specific order of what to do.
There are "Past" and "Future" posts that, when hit, will allow you to time travel to that era. Just run fast enough for long enough and you'll blast to the past or go back to the future. The trouble is that you only have shot to do it. If you start running and then get hit, you blew. Gotta find another post.
To get the good ending, you have to go the past, destroy all the robots, go back to the present, and then finally end up in the future. Waaay easier said than done.
There is definitely a good game in Sonic CD, but the main draw is so complex, many players will be turned off to how bizarre it can be.