The people who have given this game less than 5 stars do not know what they are talking about.
The Campaign is awesome, will take you 14 hours or so on Hardcore and can be played with 4 others and there is also an arcade mode for each chapter for high scoring.
Horde mode has been upgraded where you have boss fights, use turrets and build fortifications such as barb wire, etc.
Beast mode is great fun. Playing as the different locust enemies is cool, but at the moment there are not enough waves (only 12).
The biggest improvment is the MP. I could never get into MP on Gears 2, but Gears 3 is great as it is more accessible and the weapons are well balanced. It also provides a casual versus mode where new players to the franchise or those who weren't good at the previous games MP, can now get to grips with it before taking on the main community.
Take this with hundreds of ribbons, medals and achievements to earn, along with special Gears events during each month, and the package as a whole is a bargain even at full price.
All in all the best game of the franchise.