The MMII contains alot of new monsters for 3E. With their emphasis on "tougher encounters," alot of the creatures have higher CR's. Unfortunately, it seems that the good folk at WotC don't realize that you can make an encounter tougher without using a tougher monster. One of my favorite methods of accomplishing this is by adding character classes to monsters. Kytons are dangerous- but a 4th-level kyton fighter who has his Weapon Specialization in chains and just slugged down a potion of haste is a beat-down waiting to happen, even for experienced parties.
Nonetheless, the MMII contains many monsters that simply should have been in the MM, such as the phoenix, death knight, and crimson death. They had to put some sort of limit on the original MM, so it's good to see that "missed monsters" from the older versions of the game will find a happy home in 3E. I particularly enjoyed seeing the Marrash back, one of the cooler monsters devised for 2E, and the malebranche, essential for any infernal excursion.
MMII also contains monsters collected from published adventures (i.e. mooncalf, twig blight, bladeling) and some of the neater creatures from the now-defunct Chainmail game (stone spike, felldrakes, the Naresh demons).
What I was most pleased with in the MMII is the handy tables at the front of the book, combining all the needed information that you have to flip around for when designing your own monsters. This alone will make it easier and faster for me to design my own critters.
Overall, I'd recommend this book to folks who want clarified monster design and to incorporate the Chainmail creatures. I myself would like to see the Chainmail setting published for 3E.