Why do people hate this game?
Let's start with the gameplay. Well what do you expect? It's another 3D Sonic game. Only this time, you're playing as three characters at once. Plus it's the closest to the old-school Sonic games on the Sega Mega Drive. Right at the beginning, you can select from one of four teams: Team Sonic, Team Dark, Team Rose, and Team Chaotix. Depending on your skills and your favorite characters, you can pick that team. Now the gameplay is like this: It's traditional Sonic Adventure-style gameplay, with the ability to switch between three characters, like I said earlier. You can switch between Speed, Flight, and Power formations. In Speed formation, you can run a lot faster than the other formations and you can hold down the B button to do a Rocket Accel (or whatever it's spelled). In Flight formation, you can fly for a short time and you can do a Thunder Shoot to hit nearby enemies. In Power formation, you can smash through weak walls and do a Triangle Dive. The controls take time to get used to, but once you get the hang of it, it's not that bad. Another thing about this game is the special stages. You have to find a key in a metal cage and use the Power formation to break it and get the key. The special stages remind me of Sonic 2's special stages, only it's not taking place in a half-pipe. They are you getting as many colored spheres as possible (not rings, I said colored spheres). If there's a Chaos Emerald in the special stage, you have to speed up to get it within the time limit. Like I said, it's the closest to the classic Sonic games on the Mega Drive, because of the special stages. As a result, the gameplay is pretty good.
The graphics are fantastic at its time. Sonic and the other characters look stellar and the environments look awesome. It sounds great too - the voices match the characters nicely except for Tails, who sounds like he has a cold. The sound effects and music are good as well.
All in all, Sonic Heroes is a great and underrated game that's well worth checking out if you're a Sonic fan. Everybody else is probably better off buying Sonic Mega Collection or Sonic Adventure 2 Battle first if they want a Sonic fix for the Gamecube, though.