This game is fantastic. I can rate some things here.
The Good bits
.Great Gameplay, music is absolutely fantastic, and plenty too explore through. The gimmicks are cool, and the level designs are very creative, as well as the music itself. The coolest thing has to be Time travel.
.Time attack is good fun, especially challenging the staff times. I managed to beat Collision Chaos act 1 in 48 seconds. Their time was 49 on Sonic Gems collection. It took me a while to do though.
.The challenges are great, both easy & hard such as the bosses & the time stones.
The bad bits.
.Amy didn't have much of a role except getting captured by Metal Sonic, and then rescued at Starlight Speedway act 3 so she had no point of being in the actuall game unless she had a better role than that.
.As JEV3 said, one of the biggest letdowns is final boss which had a ridiculous battle with it, having to hurt yourself to hit the first blade off, the time stone award was a bit crap, just a better ending. Too bad Super Sonic didn't make it into the final version.
.I hated Collision Chaos zone. There was too many places to bounce around, and there wasn't very many areas to blast away to your hearts content. It's all about speed, not bouncing too much before opening your sick bags at the end of it. Also there was quite a few unexpected cheap hazards such as popping up spikes, and then you go so fast into one slope, you land and them, slice! the blades fly off from them into Sonic's face from them little cricket things on wheels on. Not good.
But other from that, I really enjoy the game. It's my 2nd favourite Sonic game.
Overall rating: 9/10