Sonic Chaos introduces playable Tails to the Master System and Game Gear. But it feels odd.
It also introduces special stages, only accessible by Sonic, by collecting a certain number of rings you will automatically warp into the special stage, where somewhere in the level a Chaos Emerald is hidden. Sonic now also has rocket shoes, and both Tails and Sonic can wear spring shoes to bounce all over the level; why these weren't included in the Mega Drive games puzzles me, since they are really fun to use. There are also corkscrews like in the Mega Drive Sonic 2, and these odd slanted floors/rooftops, which require you to either run at high speed or jump quickly in order to avoid falling off; something that is also not used in any of the 16-bit games.
The level design is different to the 1st and 2nd games, both artistically and physically. First, the art is more like 16-bit Sonic 1, or Sonic CD in some respects, with colours everywhere, and a built environment feel to most levels, making the game feel a bit surreal. The Music is emotive, and feels very strange, adding to the surrealness of the colourful art. Physically, the layout has become more complex, with multiple paths to most levels, so if you want all those rings you have to explore upper and lower areas. Each zone is very different , almost to the extent that it feels disjointed.
Boss battles vary in difficulty, and not necessarily in chronological order; some later bosses are easier than earlier ones, but they all provide a different challenge. The end boss is in several stages, and fans of the franchise will enjoy Robotnik's showdown at the end.
I personally prefer the artwork in Sonic 2 to this, as it feels more natural and realistic. I also prefer Sonic 1 and 2's more challenging gameplay and raw basic approach. However, I like the end boss in this game and the extra gimmicks, so it is worth playing.