Good, but not great.
With only eight levels, only half of which allow you to blast through at high speed, and a few boss battles, make the overall game pretty short.
What really makes this game frustrating are the constant stops during levels and bad design of the levels themselves, which will have you wanting to throw your 3DS out a window.
Long-time Sonic fans will appreciate the familiar sights and sounds of past games and fans of the modern Sonic games will like having this on-the-go, and with lots of different items to unlock it will keep you coming back. Sonic Generations for the 3DS is an OK game, but I suggest waiting to buy it til' the price goes down. Definitely not worth $40.