There are better Sonic games available nowadays, but Sonic Heroes is still worth a look.
The storyline is told from four different perspectives: Team Sonic's, Team Dark's, Team Rose's, and Team Chaotix's. The basic plot is that Dr. Eggman plans to bring gloom and doom upon the world, blah, blah, blah, and Sonic and friends must stop him. This game isn't plot focused like the Sonic Adventure games were.
The graphics are okay. They somewhat look like Dreamcast graphics, but they're still colorful. The backgrounds of the levels actually look really good, especially in the forest levels. The graphics could've been better, but they're still not too bad overall.
The soundtrack is decent overall, but the main theme song is just plain obnoxious. Every time you turn on the game, you get to hear the lovely intro of the theme song and then "SONIC HEROOOOOOES!" before you can get to the file-selecting screen. And guess what? If you accidentally exit out of a file, you have to sit through the intro all over again. It makes me want to swallow a cat every time it does.
The voice acting... well... meh. It's tolerable, but it's repetitive. You hear the same bits of voice acting throughout every level. "Here we go!" "Got it!" "MY TURRRRNN!" "I can take it from here!" And of course whenever a power character punches or hits something, you hear obnoxious voice clips. "Shya, hoorah!" "Raaaawr, raaaawr, huuuuuurrrrlll!!"
Sonic Heroes focuses less on story, and it has a faster pace than the Adventure games. For the most part, it doesn't involve collecting or shooting up baddies in a giant robot suit. While I actually enjoyed some of the collecting and shooting in the Adventure games, a fast pace in a Sonic game is always a good thing. Most of the levels consist of trying to reach the end of the stage while platforming and smashing up baddies. The problem, however, is that for the most part, each team has the same level. Team Sonic's levels are challening, but not necessarily hard. Team Rose's levels are a joke. Team Dark's levels are more challenging than the rest of the team's levels. Team Chaotix's levels aren't necessarily hard, but they are tedious and may require backtracking because they consist of collecting items or defeating a certain number of enemies.
Playing with four teams wouldn't be so bad if all the levels weren't the same, and that's where the game's major issues come in. The levels are fun, yes, but it's repetitive when you're playing through them four times. The difficulty shifts and collecting missions aren't enough to stop the tedium. The tag teaming isn't a bad concept, but like I said before, there weren't enough levels to sustain all the teams. The teams needed different levels. Sonic Team could've taken the Sonic Adventure 2 Battle approach by changing up the level's atmospheres, tones, and overall design. This approach helped the levels that were in the same places from feeling repetitive.
Overall, Sonic Heroes is worth playing, but it can get pretty repetitive. If you can get it cheap, then it's worth playing. You'd be better off getting some portable Sonic games (e.g. Rush, Rush Adventure) or a nice copy of Sonic Colors. If you've already played those games, though, and you're in need of a Sonic fix while you wait for Sonic Generations, this is definitely worth looking into. Sure, it's repetitive, but the levels are well designed, and there were times when I didn't mind playing through the same levels over and over again.