I don't exactly know why people hate this game
The main plot of this game is that 4 teams each with 3 members(sonic,dark,rose,chaotix) are trying to beat eggman each for different reasons(there is a bit of a twist at the end though). The basic gameplay is meeting level objectives (which are different in each team) by changing forms from speed (to go through loops etc.), power (to kill robots/break obstacles etc.) and flying (to go over obstacles etc.). Mastering this can take a few minutes(for me at least).
There is alot of good in this game I think, some of them are: good graphics(compared to other sonic games), good team playing, meaning like changing from speed to power to break a rock(even though some people don't like this), good story mode(different story for each team), and an OK repetitive value(with extra missions and stuff). There are also some bad in this game and the biggest one I think is the camera(In some parts it's unresponsive, which caused me to die alot), The multiplayer I think isn't that good either, especially the battle one(because both players have to share the camera,so its hard to tell where you are), the last bad is the voices of the characters, which doesn't really sound like the actual people.
Besides that, sonic heroes, I think is probably the best 3D sonic game in the xbox, that's why i'm rating it a 8.5.