I firmly believe that this game is underrated. It isn't perfect, but it's pretty good for a DC launch title
You can also play as Tails, who plays like Sonic, except you can fly to make shortcuts. Knuckles's levels are completely different, you glide around searching for pieces of the master emerald, a nice change of pace, and still pretty fun. Amy, who was first seen in Sonic CD, holds a hammer and has to run away from a giant robot. Her missions are fairly entertaining. You can play as a robot, who has a gun with a missile lock-on, reasonably fun, but starts to stray away from the Sonic gameplay. Lastly, there are 4 missions where you play as Big the Cat, who fishes his pet frog out of the water....um, yeah. If you beat all of them...well, you'll see.
Graphics: Very nice, bright, smooth, and fast. The game looks great, only issue lies with the camera, fortunately you have some amount of control over it.
Sound: All the familiar sound effects, like Sonic's "booweep" when he jumps. The soundtrack is hit or miss, depending on your tastes. Either way, it fits the gameplay. Some of the voice actors are pretty bad though, Tails especially. The dialog and plot in general are not good, but story isn't why you play the game
Overall, if you are a Sonic fan, you should own a Dreamcast and get this game, as well as its sequel. Both are extremely fun, and do the series justice. The game is reasonably long, and has enough extra missions to spend a lot of time on.