As a Sonic fan i bought this game the day that came out here in Europe and... I have mixed opinion on this game.
The story: it was good, but a few things weren't explained: what is the Lost Hex?? Where the deadly six came out?? How Tails knew about the Lost Hex at the beginning of the game??? The game doesn't explain these question. The writing was really good and i'm happy the dialogue are better than the one in Generations, but when there cutscene that focuses on the deadly six you just want to play the f****** level.
Graphics: this game looks beatiful, not like Unleashed or Generations. The model are done really well, every area has a distinctive atmosphere and there is nothing else that i can about it.
Gameplay: it's nothing like the Unleashed trilogy: the boost is gone and we have a gameplay style similar to the Adventure games. Sonic still has the homing attack which can attack more than one target and can also be charged making every boss a complete joke. The double jump from colours is back, he can also perform the bounce attack from Adventure 2 and the spin dash is back. The differnce beetwen this game and the Adventure games is the fact that Sonic has two different speeds: a walking animation and a super peel-out. This game's gimmick is the use of the parkour system: if Sonic runs into a wall with a little bit of speed he will climb that wall. The first time can be hard to use it, because the game doesn't tell you how to do that. Even if you read the hints, they won't help you that much. The only way to know how to use properly is via experimantation or by watching someone on Youtube. The stage's themes got more like a mix beetwen the New Super Mario Bros and Super Mario Galaxy. Most of the levels are really fun, but they don't feel very "Sonic". There aren't many loops in this game and some stages are really flat. The only levels that i can't stand are Sky Road act 3, Hidden World act 4, Silent Forest and especially LAVA MOUTAIN ACT 2, f*** this level. The bosses are really easy: like i said before, with the charged homing attack they go down really quickly and the final boss is easier than the boss fight in Sonic Colours. Laslty this game brought the wisps back and the new update we can controll them all with controll stick (BTW the bomb wisp sucks, so don't use it)
Sound: the music in this game is fantastic. Every level has a music that fits with the level theme, like a jazz music for the casino level or a more tropical music for Tropical Coast. The Deadly Six's theme is my favourite Sonic's music of all time and the other boss are really good. The voice acting is also great, so i don't have any complain with it.
Final Verdict: it's not a great Sonic game, but it's one of th funniest game. With good story, great music and graphics and an ok gameplay, i think you should buy this game... when the prize gets cut down a little bit. it's not worth 40-50€ or $.