The best looking thing graphics-wise is Swirly Snake, the Sega Swirls mascot. He sits in the bottom-right corner of the board and reacts to your score. The music that's played during a game is very soft, nothing that noticeable. The music played on the mode-select screen is very cartoony and can get annoying after a while. The sound effects are average.
Different sounds are played depending on the size of the combo, a different sound effect is played.
Although that's not much, it's a lot better than just one sound effect being played for every combo. Overall, Sega Swirls is great. The game is very complex and takes skill to play and actually be good, making the replay value very high. I highly recommended Sega Swirls for any Tetris fan. Besides, the game is free, there's no reason not to get a free game.