Name a 2D sonic game and it is probably very, very good. Sonic Chaos is no exception. Since the Game Gear processing hardware was essentially limitless (for the time) Sonic was ported over easily as the pinball wizard he is. Tails hitches along for the ride as well, sans flight. But dig past all of the Sonic-ness and you will find a deeper game than the classic Genesis titles.
Before I start touting Sonic Chaos's uniqueness, I must point out one thing: the Game Gear screen was not meant for this. While the huge view is reasonably adequate, the trippy motion blur is not. Sonic is simply too fast for a screen of this nature and many an accidental death may occur as a result (the size is partially to blame for this, but for the most part, as stated, it is a non-issue). Anyway, that minor gripe aside I say we start on what makes the game great.
While Sonic has always had great 2D adventures, they were never really interesting. Like I said, it's pinball not platforming. This game understands that pinball wont work quite as well on the Game Gear screen, so they added a little something that Mario had for years: level design. Not that they dropped the sense of speed, they simply lowered its importance. Lots of new level elements have been included, such as a puzzle where you must bounce on a moving spring and destroy certain blocks with your head (Mario, Sonic style) while moving upwards. Another nice touch is that Sonic and Tails aren't very different in how they play (which is a little disappointing) but while Sonic's levels are a little easier, Tales gets a bunch of continues right off the bat. There are also a few new powerups, like rocket shoes (like the flying kind).
There was one thing that they changed which they didn't need to which really bugged me. I love boss battles and while they were never a huge part of Sonic, Eggman just should be the boss every time. He isn't. Instead we get 2-bit robots. This is a minor gripe, since the battles aren't all the different from what you would expect (excluding a few hit-detection issues) but removing Dr.Robotnick is just ... wrong ...
Despite these few issues, this is a highly enjoyable portable experience, which is worth the cost in batteries just to see what Sonic's designers can do when they are thrown a curveball like the Game Gear.