This is a great collection game! Not as good as Sonic Mega Collection, but still lots of fun!
This game consists of: Sonic the Fighters, Sonic R, Sonic CD, 6 Gamegear titles which didn't appear on "Sonic Mega Collection Plus", and Vector Man 1 & 2.
Sonic the Fighters: This game reminds me of a simplified, comical version of Tekken. (Of course, the graphics are much nicer than the PS1 Tekken games.) You choose 1 of 8 characters to fight through all stages and defeat 2 final bosses at the end of the game. Each character has strongpoints and weaknesses, so you can pick whichever character suits you.
Sonic R: This game has 9 characters (including Super Sonic) which are playable in this game. Although there are only 5 tracks to choose from, they are very detailed and have many different paths you can take. There are other fun challenges you can take on in any of these stages. This game allows up to 4 players with a smooth frame-rate and you may choose racing or balloon hunt in multiplayer mode. Each of these characters has their own special abilities, which makes this game worth playing again and again. The music is superb and you get the option of hearing vocals with the tunes or just to hear the tunes by themselves.
Sonic CD: This game plays like the old-school Sonic Genesis games. Only game features CD music rather than the midis you hear in Sonic's other side-scrollers. The level design is amazing and has multiple paths you can explore. Yes, this game has all the speed, thrills, and graphics of a good Sonic game. In addition to all of this, Sonic CD has cool anime clips in it, which are quite rewarding for starting or beating the game.
The other Game Gear games are: Sonic the Hedgehog 2, Sonic Triple Trouble, Tails Adventures, Tails Skypatrol, Sonic Spinball, and Sonic Drift 2.
The Genesis games are Vector Man 1 and Vector Man 2.
So whether you like racing, fighting, action/adventure, shooting, or all of those, Sonic Gems collection has something for you.