I wouldn't say it's a bad platformer game. But I can't say it's a good Sonic game either.
Sonic Heroes is no exception. Even tough it doesn't live up to the previous Sonic games, it's a good enough of a platformer to keep to entertained.
There are big problems with this game. First of all, the videos suck. There is no story what-so-ever that keeps the levels of the games together.
Then there comes the team work. It's kind of confusing to control 3 characters at the same time and you might end up dead because of this.
Speaking of dying, that's the easiest thing to do in the game. Sonic is the speed characters and, if you let him, he'll run like hell. Sometimes the floor simply comes to an end, and if you're going to fast, you'll fall straight down.