It's... okay.
The Good: The layout is great, the art is nice, it looks like it's really easy to use without any prep (something most of the other MM have lacked). Variant monsters are a great idea and seem well implemented.
The Bad: There are very few hooks in the monsters. I don't know about anyone else, but when I buy a monster book, the flavor text usually inspires me to use a monster. That's why I loved the 2e MM, the 3e FF (which had lots of cool ideas), and the 2 Monsternomicons. All of those are just packed with cool ideas and the 4e MM... just isn't. The creatures all look easy to use, but nothing in the book makes me want to use them.
It's missing some classic monsters.
I don't like that monsters are now basically just a collection of combat info. Sure, that's important, but I liked it when they had some weird non-combat abilities. Now everything seems only useful in a fight.
Also, I dislike the fact that there aren't any real descriptions in the book, just pictures. And that templates are in the DMG. Whu?
Consensus: Very good book for actual play, but it doesn't spark my imagination at all. To this day, I still pull out my old 2e MM and read it every once in a while. It's just fun to read. Horrible to use in actual play, but fun to read. Now, the various 3e books seem like a good balance between the two. It's too bad the 4e MM couldn't hit that same middle ground.