Sonic Heroes is easily the best 3D Sonic game to date.
Sonic Heroes has two acts per zone with seven zones. That's 14 great fun levels in the game. Each zone also has a fun unique theme to it. There's a zone with a green hill time style to it, a cool chemical plant thing, pinball and casino levels, levels based around riding rails among others. The gameplay is based around using 3 characters at once, and it works really well. It allows the gameplay to be fast paced as well as have some great platforming and light combat as well. You'll switch between characters to change gameplay styles from speed, power and fly. Changing to fly allows you to make bigger jumps and reach higher places. Power allows you to break through stuff and beat enemies. It all works effortlessly. It allows gameplay variety with speed, good platforming and light combat and better still it's actually fun.
The game isn't very long, but Sonic games aren't meant to be. Most 3D Sonic games add too many terrible filler levels that make you want to cry(I'm looking at you Sonic Adventure 2!). Heroes knows what it's doing with it's gameplay. You do have four stories to do, but they all play pretty much exactly the same. Team Rose is shorter, easier and slower than the other stories and Team Chaotix is a mission mode type thing. The bosses in the game are a bit lame though.
My biggest gripe with the game are the special stages. They're awful. So bad. The controls are really bad for the special stages and they're way too hard. Plus you need to complete a level with a key to play a special stage, which you can only play once. If you fail that once then you would have to replay an entire level again to get to the special stage again. This wouldn't be a big deal if it weren't for one thing. You need all the chaos emeralds to unlock the true ending. To this day I've never done that or even come close to it. It's not a game breaker, but it is quite annoying. Again, when I compare the annoyances in other 3D Sonic games to this, I just thank god this is the worst thing about this game.
The level designs are actually really good in this game. By far the best level design in any other 3D Sonic game. You're constantly seeing something new and interesting. Plus the game can get quite challenging in the later levels as well as being great fun.
In terms of replay value, it's hit and miss. It's hit in the sense that the levels are really fun to play and there's plenty of emblems to try and get, which involve beating the clock, defeating a certain number of enemies, collecting a certain amount of rings and doing missions. But it's miss in the trying to better your score and rank sense. The levels are a little too long for this. Levels being a good length is great, but when it comes to replaying a level to get a better score the levels go on for too long. Sonic Unleashed is definitely the best 3D Sonic game for replaying for a better rank.
The music in Sonic Heroes is outstanding, as is just about every Sonic game. The tracks are nice and catchy and give a good nostalgic sonic feeling. I think this is probably the best 3D Sonic soundtrack ever. Well Unleashed does come quite close though. Actually I take that back. Sonic Unleashed and Sonic Heroes are about equal when it comes to the soundtracks.
Sonic Heroes is a great platformer and easily Sonics best 3D outing to date.