Famitsu has reviewed Metal Gear Solid V: Ground Zeroes already and the score is very high.
The four reviewers of Famitsu scored Metal Gear Solid V: Ground Zeroes a 10/10/9/9. This gives it a total score of a 38/40. Very impressive for a game that is only a prologue to The Phantom Pain.
Some fans were worried that the game would be too short because of its 2 hour length. If it is indeed that length, it sounds like it will be an excellent 2 hours judging from the high score it was given.