Seriously don't bother.
What was ironic was for years it seemed like Sonic's best game's were handhelds like Sonic Advance and Sonic Rush but with the 3DS version of Sonic Generations it seems we have a 180 degree shift. This really feels like something Sega quickly cobbled together because they wanted something for the 3DS for Christmas. My friends on the Sonicspot board often refer to this as "Sonic Rush Space Time Continum Adventure."
Storywise, it's the same plot as it's console big brother but more watered down only featuring the 2 Sonics, Eggman and Tails. Metal Sonic, Shadow and Silver show up for the rival battles still but the story is told with static text cutscenes with copy/pasted images from the console cutscenes. If like me you played through the console version, you'll more than likely skip over these.
Gameplay the classic stages are not that different from it's big brother accept Dimps wanted to give retro Sonic a homing attack. This didn't bother me but if you were one of those who had a fit over it in Sonic 4 then you'll hate it. Modern Sonic is basically just Sonic Rush again and if you played both Rushes and the DS Colors, don't expect anything different. Also after beating both acts, you play a special stage based on the tube from Sonic Heroes where you chase the emerald collecting bubbles to speed up. This is clearly to show off the 3D effect. Though in fairness they're way easier here than in Heroes and you can restart when you mess up.
Then you have the afformentioned rival battles with Metal Sonic, Shadow and Silver and these are basically taken from Sonic Rivals (remember those games? Neither do I?) Then the boss fights and getting all seven emeralds unlocks the fight with the Time Eater.
If there was one reason to play the 3DS Generations if you already played the console versions it's that it does feature stages different from the other version and different era bosses. Green Hill, Casino Night and Mushroom Hill from the Genesis era. Emerald Coast and Radical Highway (which if you recall was a stage played by Shadow not Sonic) from the Dreamcast era but the Modern era only features Water Temple from Sonic Rush and Tropical Resort from Colors. Seriously no representation from the Sonic Advances or Sonic Rush Adventure? Making this game shorter as well.
The game also has missions but they're totally optional and are only there for you to unlock the character art and music and of course you can compete for high scores via Street Pass.
While the game is not bad, it's very forgettable especially after playing Generations on the PS3/360. Once I beat it, I didn't have a desire to return to it much.