I'm going to go ahead and assume you know what the Monster Manual is for...
Exceptionally good presentation, lots of useful snippets of information designed to give you ideas of how to use the various monsters in your game. Plenty of illustrations of a good quality. There are encounter ideas at the front of the book as well.
The addition of 'legendary monsters' rules to make that gigantic dragon battle all the more serious are one of my favourite things to appear in this edition of D&D - essentially, truly legendary foes get certain additional actions in between the turns of the puny heroes.
Does not have 5 stars because of a 'crunch' let-down in that it is missing a few useful features, such as lists of monsters by type, by Challenge rating and so forth. At least one of those is a free download from the WotC website - this isn't good enough, it should have been in the book!