Sonic 3D is a mediocre addition to the Sonic franchise which tries a totally different type of game play.
The story goes that Robotnik learns about the Flickies and decides to turn them into robots to do his evil deeds. When Sonic arrives on the island to see his little birdie friends like this he leaps into action to free them all and thwart Robotnik's plans.
When you get to the game play you'll learn that Sonic 3D is not 3D at all, but isometric to give a 3D feel. I guess Isometric Sonic or I-sonic-metric just didn't sound as appealing. LOL. Anyway this is very different to any other Sonic game you may have played. There is no empthasis on speed and not many platform sections. There are 7 zones each with three acts. Acts one and two involve saving Flickies and Act 3 is always a boss. You run around an area and have to smash all five robots and free the flickies and lead them to the giant ring. When you do this, you will be taken to the next segment of the level.
When there are a few platform sections in the game it is really hard to judge weather or not your jump will make it. Often I jump and go too far or not far enough and miss the platform and have to try again. The jumping being hard to judge also makes bosses last longer than they need to and makes some robots hard to destroy. You are better off using the spin dash to take out robots in the levels.
To get Chaos Emeralds in this game you have to collect50 rings in a level and find Tails or Knuckles and give them the rings and you'll be taken to the special stage. These are played on rails (ie. Always moving forwards) and you run across a bridge in a volcano or above the clouds depending on who you gave the rings to, and you will have to collect the specified number of rings whilst avoiding the spikes. This is really easy, you should pass every special stage first time without even trying.
One problem I have with the game is that it's too long to finish in one sitting, I've only completed it with Mega Collection Plus' save feature. Maybe I just suck at this game but I can't see how anyone can play a game which gets boring quickly for 1 hour and 50 mins.
Graphically the game does have some pretty impressive backgrounds. The sprites look a bit weird and the floor you run around on is all just tiles, but overall the graphics are pretty cool.
The music, I'll admit, is some of the best from Sonic's glory days. While granted not all the tracks are great there are some that really shine. I frequently used to turn this game on just for the music. And just as a bit of trivia, Green Grove and Puppet Painc's songs were remixed in Sonic Adventure for Windy Valley and Twinkle Park! Awesome!
Overall Sonic 3D is a mediocre game, you can get a kick out of it sometimes and the music is fantastic. If you're going to play this, I suggest you do it on the mega collection games because if you don't like it, those games will have other things to keep you occupied.