Sonic Adventure DX is one the most enjoyable games, that I've ever played. In terms of shear content, it's loaded. And it's so Sonic through and through. Originally a SEGA Dreamcast game, this is the Gamecube "DX director's edition" which means it has improved graphics and a TON of added content.
You get 6 characters to play the game through;, Sonic (Who goes through the levels regularly going from point A to point B like in the original games), Tails (Who usually races Sonic to the exit of his levels), Knuckles (who usually has to search for 3 pieces of the shattered Master Emerald), Amy (Who must escape from a robot who's trying to catch her in her levels), Gamma (who just blows stuff up) and Big the Cat (who has the most unique mode because his levels are a fishing game). The characters stories intersect, each telling part of the larger story at hand from each of their perspectives (basically 'ol Eggman's after the Chaos Emeralds again, this time to power up some water creature he discovered.)
Beyond the adventure mode, there's a mission mode, where you find clues laying around the Hubs (like "What's under the lion's paw?" or something like that, and you have to figure out where to go to accomplish your task.
There's also a virtual pet thing with little critters called "Chaos" which I don't personally get, but it is cute and I'm sure lots of other people enjoy it.
Then, there's also a bunch of old Sonic Game Gear games unlockable everytime you collect 20 emblems (either laying around in levels or you get one everytime you beat a level).
Like I said there's tons to keep you occupied in this game, and I didn't even mention that there are 3 objectives per level for each character in Adventure mode!
Well, the content alone gives this game an "A". But as an added bonus the game looks and sounds great, with amazing graphics, fine music and classic sound effects. Plus everyone gets their own "theme song" the funniest of which is Knuckles's "Unknown from M.E" which is basically Knuckles rapping.
Anyway, this game is awesome, and even though it's an old title, if you see it collecting dust at a second hand store, or something give it a shot. Sonic fans of all ages will love it.