It felt like a massive step down from SA2B (my favorite Sonic game). In terms of the levels and definitely the story.
Also, Sonic Heroes originally was only going to release on the GameCube and XBox. Sony "forced" them to make a PS2 version. The time to make this version cut into time that could have been spent making the game better. Also, the PS2 version plays the worst, so avoid that version.
Also, it preceded Shadow the Hedgehog and Sonic 2006, so I guess you can say this is where Sonic's quality took a dive it took a while to recover from...and the esteem of the franchise still suffers to this day from this.
I'm...the...master...of...ellipses...