Right after 100% completing Sonic Generations, I decided to get high and replay this game on dolphin... Here's some random unorganized thoughts I wrote down as I played through Sonic's story.
This Sonic is in a weird mid range between classic Sonic and modern Sonic... he's a bit of both.
The first couple levels are very easy to find in the hub world... nothing much to say about them. It's immediately noticeable how bad this game really is in comparison to the newer 3d Sonic games like Generations and Colors... The music is so good though, it's probably the main thing that made me love this game back then.
Casinopolis is the first level that's a bit tricky to get to... it involves finding the lightspeed dash ability that is randomly found in the sewer...
Found this cool card pinball minigame with a NiGHTS cameo... and also a slot machine pinball area like in Sonic 2.
The lobby/shower area is weird... Every time you come out of the pinball zone it fills up with rings and you have to go through a short action/platforming section to get back to the lobby, until you deposit enough rings to get to the chaos emerald.
Sonic: "Hmm, pinball"
yes...
Took ice key to the west cave in Mystic Ruins
Icecap... this level has some old school platforming, it really feels like a classic Sonic level, with some slow parts mixed in.
These levels are easy on the "C" mission, which is just "get to the goal ring/emerald", but the later missions are quite a bit more challenging...
So Eggman is in control of this Chaos character... in other words... he has... Chaos Control?