1996 was an awkward time for video games. 3D was the huge new thing, and everyone had to adapt or die. Some franchises made the jump quite well (see: Mario), some didn't do so to well (see: Castlevania). Sure new IPs came along much well suited to this 3D thing, but we what we really wanted was our favorite characters to make the jump. In '96 Sonic made the 'jump' to 3D... on the Genesis. The problem with this is in order to be '3D' on the genesis, is it had to be in an isometric view and imply depth instead of actually have it. Sega used this technique back in the 80's with Zaxxon. Later on Sega ported this game to the saturn with enhanced graphics, and that version was ported to the PC. I'll mention the differences as they come.
Gameplay wise its still a platformer as you would expect with Sonic, but the formula has changed quite a bit with the new perspective. Sonic does no longer run and twirl his legs, he just jogs. My first experience with this game is the PC version, and as a result I recommend you do not play this game with a keyboard. Keyboard controls with the isometric perspective are impossible. Not to say a lot of the jumps aren't agrivating enough with the controller. You have a little shadow below you to see where you're gonna land, but that only helps so much.
To make the most out of the isometric levels they've made them somewhat non-linear, by forcing you to collect 5 birds (called Flickies) before you can proceed to the exit. Most acts of a level are split into 3 sections of bird collecting before you're done. The level-act system is much like previous Sonic games -- Act 1 and 2 are regular levels, act 3 is a boss level. The special stage in all versions is conceptually based on Sonic 2's, where you auto-run forward and have to collect rings. The Genesis version is the wrost because all of them are completely flat, and are entirely too easy. The Saturn special stages are great! They're fully 3D (since they're on the saturn n' all), add to the Sonic 2 special stage formula, and are just fun to play. They do get pretty challenging too. The PC special stage (yes they all have different special stages) are boring again. Once again they're full 3D but the levels are... barren. And these have an entirely different problem with difficulty -- they're too damn hard!
Everything in this game is pre-rendered. In the wake of Donkey Kong Country this is really no surprise. Compared to previous games everything looks very smooth. This really helps with the look in the mechanical levels, but in green groves and ruins not so much. The Genesis version is heavily dithered to make these graphics work, but the Saturn/PC version is much more colorful with much more detail. Not just more detailed colors, there's lots of background details added, and even fog to the second level in parts. The levels follow all the Sonic tropes you'd expect -- Grassy, ruins, carnival, industrial, etc. Yawn.
Saturn, PC, Genesis respectively. Screenshots courtesy mobygames.
The music is the standard Sonic fair, and there's some really catchy stuff here. However, there's some really generic stuff here too. Both versions feature different music, but both have their standouts/generic tracks. Of note on the Genesis version, one of the music composers, Jun Senoue, has done a lot of work for Sonic Team. As a result, some of his music from this game was reused in Sonic Adventure. He did not compose music for the Saturn/PC version, however.
Overall, I'd say Sonic 3D is a mediocre Sonic game, but an average isometric platformer. (It certainly wins out over Snake Rattle n' Roll)