And, while it's not perfect, Sonic Adventure 2
offers a lot of gaming bang for the buck. Sure, it uses the old 'do a level over and over to get a perfect rank' ploy, and the Knuckles/Rouge levels can be more annoying than fun, but the Sonic/Shadow and later Tails/Eggman levels more than make up for it.
Sonic Adventure 2 is broken up into two stories--Hero and Dark. Each story has over a dozen levels, each featuring a particular character from a given side. The various characters 'pair up': Sonic and Shadow's levels are the same style (classic Sonic speed in a 3D world), Knuckles and Rouge have sprawly adventure levels where you have to find pieces of the Master Emerald, and Tails and Eggman pilot mecha-type-things that blow up everything in their path. The characters are predetermined for a given level, but you can always go back and revisit the levels you've already played.
There's a plotline here, but I got confused about halfway through the game and just kept playing for playing's sake. Shadow is taking over the world and Eggman is helping and the Moon gets blown up and all sorts of wackiness ensues. I don't know; it's a platformer. I generally ignore storyline for these sorts of games.
The lowest points of the game are the Knuckles/Rouge levels. While the concept is intriguing at first--it's basically 'hot/cold' with enemies and much larger spaces than your house--it ends up being rather interminable. You'll run around an absolutely massive (and gorgeous) world, trying to find some crystals. Sure, you can use the hint machines, but then your rank goes down. And you still have to learn the worlds to be able to understand the hints that they give. Ugh. I found myself wishing I could just skip these stages, but I persevered and got through them.
Higher on the evolutionary scale of fun are the Tails and Robotnik levels. While the first time you play each of them you'll feel a little bored--aim, seek, and shoot--as the levels progress, their stages get more and more challenging. You basically run and gun your way through a sequence of passages/walkways/whatever; your weapon can 'lock on' to multiple enemies, which is how you increase your score and stay alive. In addition to losing your rings when you get hit, you lose energy; get hit too many times and you explode.
The heart of Sonic Adventure 2, however, lies in the Sonic and Shadow levels. They're blisteringly fast, well-designed, and a heck of a lot of fun to play. Nothing is more entertaining than bouncing your way up an absolutely gigantic tree, or running through a crazy loop, or flying through the air and doing all sorts of crazy stunts. These stages smoke, and they're a blast to play. All of your classic Sonic action is here; spin into enemies, fly over gorges, run through crazy passageways.
There are a number of features outside of the main game--a kart affair and a Chao-raising sim--but these aren't the sorts of things I look for in a platformer. The action is where it's at, and Sonic Adventure 2 has plenty of action.