This is one of the most under-appreciated games I've ever played. It's so much better than people give it credit for.
First off, this is by far the fastest Sonic game I've ever played. You really feel the speed, especially when playing with Sonic. The controls are fairly decent. I found it a little hard to control some of the parts of some levels with Sonic where you have to run like on the beach level when you have to chase after Dr. Egg man's ship and on the burning city level when you have to run away from the giant fire tornado. Those are the parts where you go extremely fast. It's a little hard to control but it's not unplayable by any means.
The story is good, time travel is a big part of it. All of the cut-scenes in the game tie the story together, and if you play through Sonic's, Shadow's and Silver's stories you unlock a final chapter.
There's a good number of playable characters. Sonic, Shadow, Silver, Blaze, Tails, Knuckles, Amy, Rouge, and Omega. The voice acting for each of them is also done quite well.
The boss battles are pretty fun. If you play Sonic's story you fight Dr. Egg man at the end, play as Shadow and you fight Mephiles and if you play as Silver you fight Iblis.
Once in a while the camera can be a little off, making you die when it's not really your fault, but it doesn't happen all the time.
The graphics are also really good, the best I've seen in any Sonic game so far.
There's different missions you get to do in between playing through the story mode. They're completely optional but if you do them you earn extra rings to buy upgrades with, and you need a lot of rings for some of the upgrades. The only bad thing about doing these mission is that there's too many loading times in between. The loading times can get pretty annoying but they're usually not longer then 10 seconds.
I haven't been able to try Multi-player because I only have one controller, but I'm sure it's also pretty fun.
The soundtrack is awesome, it has some of the best songs I've ever heard in a Sonic game. His World (Sonic's theme) and Dreams of an Absolution (Silver's theme) are really good songs.
For the most part it's a hard game. Some levels are easier than others but they do take a while to complete. Especially with Silver, it took me 33 minutes to do one of this levels.
It's also a little weird why they decided to "pair up" Sonic and Elise and Silver and Blaze. A hedgehog and a human can't mate, neither can a hedgehog and a cat. It just shouldn't be possible. Sorry for bringing this up but if you've played through the story modes you'll know what I'm talking about. Maybe they weren't implying it like that but you can at least tell they made it seem like those characters like each other. In one cut-scene Amy talks to Elise and you find out that Elise is in love with Sonic. Silver and Blaze aren't as obvious but you find out in the final cut-scene of his story, so yeah.
Another great thing to mention is that this game is now ONLY $19.99. That's such a great deal.
I'm not sure what else to say really. If you're a Sonic fan you'll likely enjoy this game. It's a very under-appreciated game and I was really disappointed with the reviews and scores it got.