Insanely good, return to supersonic roots
First, Sonic is not a long game, so if you are looking for a lasting adventure game, go some where else. This is all about speed. Sonic and second playable character blaze move so fast its a wonder your ds doesn't explode.
The levels are well designed and get your heart pounding. There are classic sonic zones remixed for the ds. There are some issues where you run right into enemies but it is not so bad if you continuously use boost.
To get boost you use a surprisingly deep trick system. By tapping the r button on rails and mashing the b and a buttons in midair (only off of "gimmicks" not your own jumps) your get more boost.
Which, when used, lets you blow through enemys.
The music, is very well done and will get stuck in your head. They used pseudo words and sounds instead of lyrics in most songs, but they move a such a clip that even if there were words you coulden't hear them. The sound effects are standard sonic fare with all the expected, blings and boings.
The 2.5d bosses are very well done and get challenging later in the game. (Just wait until the monstrous last boss.)
Although the main story mode is short, and sonic and blaze are basically the same, the are a few extra modes that you get once you beat the game. The time attack mode lets you race in the levels and improve, by seeing your ghost, Mario kart style.
Sonic rush, is a fast, fun platformer that requires ninja reflexes.