For a Sonic fan, it's fun. For a nuetral gamer, it's probably just rentable.
Gameplay:
Sonic Heroes uses a team system. The four teams - Team Sonic (Sonic, Tails, and Knuckles), Team Dark (Shadow, Rouge, and Omega), Team Rose (Amy, Big, and Cream), and Team Chaotix (Espio, Vector, and Charmy) - are separated by difficulty. Team Sonic being normal, Team Dark being hard, Team Rose being easy, and Team Chaotix having special missions.
In order to complete each level, you must switch between the different types of characters. The "speed" characters (Sonic, Shadow, Amy, and Espio) are used for basic running. The "flight" characters (Tails, Rouge, Cream, and Charmy) are used for flying over gaps and for reaching high areas or attacking high enemies. And finally, the "power" characters (Knuckles, Omega, Big, and Vector) are used for basic combat and breaking through barriers / walls / etc.
You must constantly switch between these characters in order to complete obstacles and finish each level. It's safe to say that this system works very well, and isn't overly complicated or beyond mastering.
Graphics:
Although the GameCube is capable of stronger detail, the graphics are still good. One disturbing flaw is that all of the characters are very...shiny.
Sound:
Like all Sonic games, Sonic Heroes suffers from bad voice acting, as well as a cheesy script. The theme songs for each team are mediocre, with the exception of Team Dark's "This Machine" song, which is arguably the only good song on the game's soundtrack.
Value:
The multiplayer isn't special, but collecting all emblems and "A" ranks will keep you playing for some time.
Sonic Heroes is still quite a fun game, especially for the Sonic fan. Others who may not be interested in it may want to rent it and make a decision for themselves.