Well, I went and looked at it over lunch. I doubt I'll buy it, but it's better than I expected.
First, the format. I don't think it's ideal, but it does look like it'd be a real help in GMing. I approve of WotC trying new things in this area, though I wish they'd keep some format -- any format -- for more than one book.
Now that I've seen all the Spawn, I have to say that they're about what I expected. Which is to say, they suck. There were one or two that would make me feel like a complete tool to use. And the backstory is lame to the point of causing physical pain.
One thing I will say about the Spawn is that they would probably be really, really fun to use in the DDM game. Which is an excellent reason to not put them in a Monster Manual. That's exactly the sort of mini-centric thought that is driving me bonkers with 3.5E.
While the maps were well-drawn, they didn't really seem appropriate to a critter book. I expect one-off maps to be in something like Dungeon, not a "permanent" resource.
My attitude on the pregen NPCs is similar. Plus, they took up too many pages. In flipping through the book, it seemed like I was in "orc" forever. The lizardmen didn't seem real great, either. Surprisingly, the gnolls were interesting. And, of course, the less said about drow, the better. Overall, I'd rather they hadn't included them.
There were quite a few really good monsters, though. The Deathdrinker demon looks very cool. It's probably the one critter I most want to use.
The additional fey looked worthwhile, too, though I didn't spend too much time reviewing them. They would definitely help with setting up a Seelie/Unseelie realm/war/etc.
The Tomb Spider was a big surprise. I fully expected to hate it (like I did with all the other bugs in the book). Instead, I found it to be an interesting entry that I could see using.
There were, of course, some real dogs, but there are in pretty much any monster book.
I think the biggest weakness of MM4 is that it didn't know what it wanted to be. New monsters, pregen NPCs, maps, or a themed (Spawn) book. It didn't really do any of those well, mostly because of space. It could have been a really excellent "Monster Manual", but that part of it is too small for the money.