Physically, it's a nice looking book, 352 pages (compare with 316 pages for the Player's Handbook). It's got the same glossy cover and half-glossy back (which is a little annoying because every time I pick it up I have to check to make sure I haven't grabbed a piece of paper beneath it by accident). It has a couple of obligatory "what is a monster" and "how to use this book" pages to explain the various monster entries, and then off we go with the aarakocra. There is ample artwork throughout (no boobs, even on the succubus and marilith), and I suppose it makes up for the lack of a text description of each creature.
Some oldies-but-goodies have been renamed. Ogre magi are now oni, which I suppose makes sense, but the rename seems somewhat arbitrary, although it does keep them right after "ogre" alphabetically. Merrow, on the other hand, have been completely changed from aquatic ogres to a corrupted form of merman. Fine. Daemons from 1st edition remain Yuggoloths, but they will always be Daemons in my game. Interestingly, although there is mention of demon lords and archdevils, there is no mention of the daemon lords or the Oinodaemon, just someone called The General. Anthraxus will, needless to say, exist in my game.
There's lots of background text, which is both a boon and a bane. It certainly will help DMs who need some help integrating a creature into a game, but in some cases it becomes intrusive, such as the entry for shadow dragons, which is replete with references to "the Shadowfell" whatever that is. Perhaps it's something that will get explained when the Dungeon Master's Guide comes out. They'll be native to the Plane of Shadow in my campaign, where the Material casts its shadow from the light of the Positive.
I know a lot of people are bemoaning the lack of a table showing each monster by it's challenge level, as they feel it would be very helpful for writing encounters. Apparently that (as well as encounters by terrain type) is coming in the DMG, just like it was in 1st edition.
Bottom line, this is a great monster book, and I think it's worth the money. You could use a lot of the background text for any game, and the selection of creatures, while not perfect (in my completely subjective opinion), is certainly defensible for a "baseline" monster book that doesn't strive to be completely comprehensive (although there is certainly room for such a thing, ahem).
This is certainly a worthy addition to the Player's Handbook, and 5th edition is certainly shaping up to be something I'll really enjoy playing and running. Next stop, the Dungeon Master's Guide!