You don't get any better (Or bigger) then this.
Story: 10/10 Depends on the game, you start with seven Genesis titles and six Game Gear titles (Unless you're on the Gamecube, no Game Gear titles there). In Sonic 1 it's stop Dr. Robotnik's mad plans to enslave every animal in Sonic's homeworld. In Sonic 2 you take Tails with you to stop the Death Egg (Miniature Death Star) from destroying the planet, courtesy of Dr. Robotnik enterprises. In Sonic 3-D Blast Dr. Robotnik heads to Flicky Island to capture little birds in his robots, little birds that Sonic has to rescue. And onwards it goes.
Gameplay: 10/10 Sonic can move left and right with the arrows (In most games, it varies by title), jump with any action button but the triangle (Varies by console), look up with the up arrow held down, and while running he can spin by pressing down. In most titles he can also spin dash by pressing down, then an action button. And, that's it. But that's enough for one of the most addicting platformer series in gaming history. You can spin into enemies to attack them, jump on trampolines to bounce high, run on loop the loops and corkscrews for speed, and more, but it's not all running and jumping, sometimes there's a simple puzzle to figure out before you can advance. That slowing down of the pace keeps Sonic from becoming a tedious run from here to there platformer. Your health is determined by rings, without any rings you'll die by touching an enemy, but with one or more you'll lose them all if you touch an enemy, though they can be picked up off the ground. Falling off a cliff or getting crushed kills you instantly, with or without rings, but those aren't major worries here due to a lot less cliffs then modern Sonic games. Boss fights are Dr. Robotnik in his little ship flying in to attack you with fancy upgrades, from a swinging bolas to a laser gun to a big hammer to miniature versions of himself, and except for some final bosses he always has six to eight health. Sonic's entire Genesis collection is here (Not counting CD or 32x titles), so you have a lot to work with. There are even a few puzzles and a racing game thrown in, and a pinball game (Though while the pinball game nad the puzzles are fun, the racing game isn't). Exclusive to the PS2 and XBOX is a save anywhere feature, which allows you to save in any game, anywhere, and leave your game to come back to it later, though it can also be abused to keep retrying a level until you win.
Music, Sound, Graphics, and Other Technical Aspects: 10/10 It's a representation of the Genesis games, so don't expect surround sound and HD, expect graphics that look pretty good still and bouncy music and you won't be disappointed.
Multiplayer: 9/10 In about half the games you can journey through the game in two player, and a few have courses you can race through.
Unlockables: 10/10 It's a collection, and there are special features to unlock. You start with the ability to look at Sonic artwork, see videos from the game Sonic Heroes, look at Sonic comic book covers, and ever read a full comic book, as well as look at the original instruction manuals for the games. But the best unlockables are new games, you can unlock 3 Sonic titles (The lock-ons from putting Sonic & Knuckles with Sonics 1, 2, and 3, giving you a puzzle game with Sonic 1, a new character in Sonic 2, and a huge game in Sonic 3 with new characters), two popular titles not related to Sonic, namely the beat 'em up Comix Zone and the gimmick platformer The Ooze, and two related-to-Sonic titles, Flicky, which stars the little birds from Sonic 3-D Blast, and Ristar, who is based on an original Sonic concept, the rabbit with stretchy arms, only now it's a star in a very fun platformer.
Overall: 9.8/10 rounded to 10/10. You don't get any better then this, 20 great (Or mostly great) titles altogether in a single disc. Unless you happen not to like the Sonic games (Hard to believe), you will love this. Other then what I've listed above the Gamecube version isn't any different then the PS2/XBOX versions, and you're not really going to miss the Game Gear titles, so don't hesitate to get it just because it's smaller, it's still one of the best games on the Gamecube.