In the interest of civil discourse, I shall share my opinion. Which, unlike a lot of other peoples, was fairly positive.
Is it my favorite monster manual? No. I think it's on par with MMIII. That is to say, lots of great ideas, but flawed execution. The MMIII's flaw was the editing - the error rate in the stat blocks was nothing short of incredible, and I'm not talking just about minor stat glitches. The MMIV's flaw is in the presentation of too many sample creatures and in the expanded entry format. Don't get me wrong. I love flavor text. I love it a lot. But do we really need a paragraph on environment? Treasure? Alignment? These things can be summarized with one line in the old stat block, and can and should be expanded on if need be in the flavor text. Not given their own paragraphs. I can tell you, writing in the MMIV standard can be annoying, especially when trying to maintain a word count.
As for the sample NPCs from MM creatures, they're a mixed bag. There's some really good ideas throughout (the abomination marshal, the ecology of gnoll tribes and Yeenoghu's "blessings", plague speakers, etc). But not enough. The gnolls were interesting. But did we need quite so much space devoted to them? Seven or so pages on one lizardfolk tribe?
Although I do admit I've already used the stats for the ogre scout in my Eberron game. So they're not useless. But not really MM material.
As for the maps, blah. Count me out.
Now, that's a lot of griping. On to what I liked. The new monsters, for the most part, are creative and interesting. There's all sorts of cool little mechanical tricks thrown out there in the book that I loved, like the blackstone triskelion being able to resist area effects by sheer fortitude and the nashrou's vulnerability to critical hits. There's a lot of concepts that excite me, like the zern (loves me the aberrant masterminds), the vitreous drinker (eyes of Vecna!), and the new demons (the deathdrinker left me a bit cold, but otherwise...). There's a lot of good monsters in here, despite there being less of them.
As for the Spawn of Tiamat... I like 'em. Kind of. They're kind of bland on paper, but feel like they'd make for interesting encounters. I've already used a greenspawn razorwing on my party, and it was quite memorable (as in, they nearly got wasted). But there's one too many of them - reading the book from cover to cover (as I am wont to do) was a little boring when I got to the Spawn Spawn Spawn section.
And I love the art in this book. Arnie Swekel is back in a big way. Even pieces that have been called out on these threads as terrible I love. He's got a great eye for detail - even a fat spider with a black dragon's head looks plausible. And Wayne Reynolds turns in some great work here, like the aforementioned vitreous drinker.
So, the MMIV is a mixed bag for me. I'm glad I bought it - I'm a monster junkie, and I'm already busily inserting creatures from it into my Age of Worms campaign. But there's some stuff in there that doesn't sit too well with me.
My ranking of the non-core Monster Manuals, in case you cared, is:
Fiend Folio
MMIII
MMIV (only slightly worse than MMIII, IMO)
MMII.
Demiurge out.