Why so many people say its bad? Its different.
First of all it was 3D (well, isometric that is) and offered a different perspective of the Sonic world. The speed was still there, but Sonic looked a bit more slower the the other sonic classic games.
The levels were good, and the colors made them more alive them ever.
The main difference in this game, was that instead of reaching the end of the levels, the objective was to rescue all the flickie birds that were inside of the robots in the levels. I don't remember how much they were but i guess more them 6?
Anyway, after you rescued the first birds, you were transported to another section of that level, and you were supposed to rescue more birds. This was a bit boring, doing the same thing, and getting those birds was hard, i remember.
Each area had 3 acts, being the last one the boss. And the boss battles were amazing.
The soundtrack of this game was also amazing, being the ice zone my favorite.
Overall, Sonic 3D was a good and a different game from the other sonics ones, but it was a bit challenging, and doing the same thing all over again could get a little boring, but still, its a great game.
p.s: i never get pass the lava boss. That was hard has hell. Everytime i tried to beat it, i always ended up losing. A few years later, finally i finished the game.