Hideous.
First off, lets talk about the graphics. Genesis Sonic games had a certain luster, a 3D like luster which was the standard and should still be the standard for 2d Sonic. Yet Sega chose to go with a cartoony look. This is called reversion. If nintendo did a NES look on SNES which was not meant to be retro, it would be reversion. Sega was going from Game Boy ADVANCE back to Game Gear with the graphics in these Sonic Advance games.
The music is pretty bad not just for a sonic game, but in general. Unlike the first 4 sonic games, the music is uninspired and unenergetic as they should be.
Then, worst off all is the gaming experience. The levels are boring, as are the badniks, and ROBOTNIK, enough of this eggman crap! The special stages are far and few in between, and just plain suck. Don't even ask about playing as the girl who HITS WITH A HAMMER! This is supposed to be Sonic, where you are fast. Amy is slow and hideous.
If you are a newb to gaming, then go ahead and waste your thirty bucks. But if you a true gamer and or Sonic fan, ignore this, and go for the original gensis games, or get em on emulator.