I was stuck between good and OK, but eventually chose good.
The Good:
The layout. I like how every monster gets a single or double page dedicated to it. (My group still remembers the day where our DM in our Oriental Adventures campaign accidentally read the Giants stat-blocks wrong and had us face a Giant way more dangerous then the intended one. It was probably our first Total Party Kill in 3E, with the Giant making one regular attack, some cleave attacks and then finishing the last man standing off with his second regular attack...)
"Variant" Monsters:
The fact that I don't have to manually advance my humanoids is a great boon. It's a nice "trick" they use - monsters you tend to use in groups (most humanoids or "cultured" creatures) exist in mutliple stat blocks around a certain level range. Monsters that you use more solitary are often available for different levels and tiers, so if you didn't get to use this monster at level 7, you have another chance at level 12, without manually advancing the monsters.
Art & Illustrations. I like most of the artwork, and of course, every monster entry also contains art (luckily, this has been the case since at least 3e...)
Knowledge DCs. That's something I like a lot - it provides information and also tells the DM how he can dispense it to his players.
Monster roles and categories. (This could also go towards the DMG)
The roles are extremely helpful in creating good encounters, and to explain how to use them effectively.
The categories like Minion/Elite/Solo are also great. Previously HD or CR was the only measure of power, and they affected critical numbers like Saves/Defenses and Attacks and hit points, so you had a narrow window to work with effectively. Minions, Elites and Solos expand the options in creating monsters and encounters.
The Bad:
Descriptions: Some monsters could be described in a little more detail. Having artwork is great, but a read-a-loud description would be fine, especially when you keep the players on their toes - showing images spoils all the mystery.
The Ugly:
I want more monsters, and some new monsters aren't that interesting - for example, I can't really warm up myself to the Kruthiks. I could probably have gotten more out of a Delver or a Phantom Fungus!