OK, I checked the 4th Edition monster manual today (or rather, the FIRST of the current monster manuals, there’s two editions out already!)
Destron mentioned how the Manual features mostly game data and not enough background material. This is mostly true. Although every entry contains “lore roll results” which tell the GM what to tell his players what they will know about the monsters depending on their skills (such as religion) which is interesting and helpful. Still, most monsters get very little explanations of their origins and goals. This is probably so they can be fit more easily into individual campaigns. Still, if there were more background material the book would make a more fun read as the older versions were (in this sense, it’s the contrary of the Manual of the Planes, which has too much prose!)
For GMs however, the book is incredibly helpful: not only it lists all the classic D&D monsters (and LOTS of them!) it gives variations of each, as well as combat data, and even typical encounter group composition, in case you don’t know which monsters should hang out together. It even has a table which lists the monsters by encounter level, which helps to know which monsters to use according to your players’ level! About the only thing I didn’t like much were the monsters’ stat sheets, but that’s probably because I’m used to the older ones. I also think that the monster variations had too many “colorful” names, as if the writers were being paid for coming up with the most flowery names. 
As for the monsters themselves, I noticed there are lots (and I mean LOTS) of new undead! And I thought I’d seen all the variations of ghosts and zombies possible already.
Strangely, while the the Nine Lords of Hell were mentioned, and all their devil minions were described, they themselves were not, which is odd because one demon prince (Orcus) was. Maybe their descriptions weren’t ready at the time the book was published. (There are a few Archdevils in the Manual of The Planes, but again, not the full nine.)
Angels also appear here. Not that they weren’t in D&D before (though not called under that name) but before they were always good, and now their alignment depends on the deity they serve. So… all Demons and Devils are evil, but not all angels are good? Doesn’t seem fair.
In fact, I noticed that there are almost no good monsters anymore; even unicorns are now “unaligned” (neutral.) That’s a pity. I understand that the main functions of monsters is to, well, be fought and defeated by the players, but I liked it when you were supposed to help some of them as well, and have more than hack-and-slash in the adventures, you know? Oh well, maybe in later volumes.
The art is the same as in the other books so far: good, though I find it kinda, I dunno, rough in the edges? Or maybe too heavily inked. At least it’s in full color, on that it beats nearly all the previous monster manuals.
Overall, reading the monster manual was very interesting. I’ll check the other one next time I’m at the bookshop.
This (hardcover) collects the best material featured in the site from 2008 to 2009. Sure it’s expensive, and technically speaking none of the material in it is “official” (that I know) but it is still, for the most part, very good and worthy of being used in many player’s campaign. Even the stuff that I felt was just “OK” was still fun to read.