Not your traditional Sonic game, but I enjoyed playing it nonetheless.
The story centers around Shadow the Hedgehog, the black hedgehog who first appeared in Sonic Adventure 2. Hence the game's title. After the events of SA2 Shadow has apparently developed amnesia, not remembering anything about his past. Then the evil alien race Black Arms shows up and start laying waste to the city of Westopolis. Their leader, Black Doom, appears before Shadow and demands him to collect the 7 Chaos Emeralds "as promised." The black hedgehog decides that the only way to remember his past is to collect the Emeralds for himself, and thus the game begins.
Gameplay: 8/10 - Shadow the Hedgehog plays pretty much like SA2 and Sonic Heroes, although the levels are a bit more open-ended. Shadow is pretty hard to control when he's going top speed, but have you ever tried pulling a 90-degree turn while going 100 miles per hour? And what's this about the guns not locking-on? It's called REALISM! What kind of real-life pistol locks-on to it's target? And the bullets do slightly home in on enemies. The controls are solid, if they do take a little getting used to.
Graphics: 10/10 - These visuals just blow me away. Whoever said that these are Dreamcast graphics must have seriously screwed-up eyes. The environments look superb, and the cutscenes took my breath away. The damage modeling on the vehicles and watching the empty shells fly out of firing weapons are very nice touches as well. The camera can be annoying, but what Sonic game hasn't had that problem?
Sound: 9/10 - I was pretty surprised when I heard Shadow say "Damn" when he got hit, but I've heard much worse at my high school. The voice acting is good, although Vector sounds like a moron in my opinion. The music really fits the mood of the game, and the sound effects are top-notch.
Replay Value: 10/10 - Each level has multiple missions: Hero, Dark, and occasionally Normal. The mission you complete decides which level you go to next, and ultimately which one of the endings you recieve. There are 10 cinematic endings, including a secret Final Ending, but there are more than 300 possible combinations of levels that you take to reach each of the 10 endings. Diehard fans will spend months adding all of the story paths to the game's Story Library.
Overall: 9.6/10 - This is an awesome addition to the Sonic series, and fans will definitely be getting their money's worth. This game's diversity will keep you entertained for hours to come.