The majority of entries in the Monster Manual have great rules, quick and easy to understand, but with many showing quirky exceptions and abilities that bely the mechanical simplicity 5th Edition otherwise exhibits. Take as an example the fiends - be they Demons, Devils, Rakshasa, Succu/Incubi, or Yugoloths - all show a variety that makes the different classes of creature really stand out from each other. While in previous editions it felt like a Pit Fiend and a Balor were, a few differences aside, much the same beast, the designers made sure that many monsters really occupied their own mechanical niches - especially at higher Challenge levels. The book also often includes variants for monsters in sidebars, like trolls whose body parts maintain their own motivations after being severed or classic Demons that can summon reinforcements from the Abyss on a whim. There are even a few of the old 3rd Edition D&D-style templates floating around, allowing DMs to make their enemies Half-Dragons or the like.
There are, though, a few uninspired wastes of a page in the book. Take, for example, the entry for the Cyclops and compare it to the entry for the Hill Giant. They're very nearly the same creature, with hit point totals very close to each other and quite literally the same two attacks doing nearly identical amounts of damage, but the Cyclops isn't very good at hitting things with ranged attacks. Aside from some remarkably similar descriptive text, that's about it. And, of course, the 33 pages spent on Dragons will thrill only a few dedicated fans, since some dragons differ very little mechanically from the each other. The legendary details provided for each dragon do somewhat make up for that sameness.
Monsters have some variance in their statistics that's obviously tied to Challenge, like hit points and proficiency bonuses, and those numbers go pretty high over the course of Challenges 1-24. However, the developer promise that target numbers wouldn't too egregiously inflate has certainly been honored. Even the most potent dragons tend to have an Armor Class and target numbers for saving throws that hover around the low 20s, meaning a well armed and powerful warrior will be able to score a few hits each round. That said, the saving throw numbers to avoid the nasty abilities and spells of the higher level monsters are rather unattainable even with their reduced range, and there's little opportunity for players who have bad saves in those categories to avoid those powers. Many players are just going to have to resign themselves to a life of eating dragon breath or petrification - which isn't a particularly appealing fate nor fun gameplay.
There are clear outliers that show how the Challenge system is as much art as science. Take, for example, the petrifying Cockatrice at a lowly Challenge 2, which can peck you for a small amount of damage and a chance to turn you to stone. Its petrification is limited to a 24 hour period, though, so since the Cockatrice deals a small amount of damage with each attack even if it "bests" the party it'll probably be an embarrassing rather than deadly encounter. The Bugbear, meanwhile, also clocks in at Challenge 2 but deals such potent damage that it will likely leave a similar party of Characters on the ground and dying. There's a certain frustration with the Challenge system being guidelines, but also a certain attractiveness.
If you're used to the regimented rules of what constituted a fair fight in 4th Edition D&D, you're going to be surprised and possibly unhappy. On the other hand, that approach combined with the limited range of Armor Class and saving throw DCs means those naturalistic players who love to think of the fantasy world as having its own rules separate from the rules of the game will enjoy the idea that though a Manticore is a potent foe suitable for third level adventurers, clever strategies could bring it down.
Carrying on the legacy of 4th Edition's Solo monsters are the much less problematically named Legendary monsters. Legendary monsters are those creatures with such fearsome reputations and potent abilities that they have always been used as cornerstones of the D&D adventures they appear in. They're things like fully grown dragons, Beholders, Sphinxes, or even The Kraken. Legendary monsters have wonderful unique mechanics that give them extra actions, neatly solving the problem exhibited in older D&D editions, where a dragon's flurry of attacks (Remember Bite/Claw/Claw/Wing/Wing/Tail/Crush?) slew a character in a round. Those powers are instead spread through the round, after their enemies act. Legendary monsters also get a few opportunities to automatically pass saving throws, avoiding the 3rd and 4th edition problem of a single "Save or Suck" spell ending the career of a potent villain.
Legendary creatures are tied to their Lairs, and they have powerful Lair Actions - like a white dragon that causes a shower of ice stalactites to fall upon its foes. In play, they're glorious, making players wary of engaging the most formidable creatures and making those climactic battles fantastically fun when they do appear. Likewise, powerful monsters also have Region Effects, the ways in which they warp the surrounding countryside to their will by their presence. Alongside the wonderful descriptions of how the monsters live, they give a real life to the D&D game world without acting as a prescriptivist description of the game's setting.
The Monster Manual is a satisfying book, and while there are multiple valid criticisms to levy against it - from mechanical oddities and range of foes, to its refusal to pin down just how hard any given creature is - there's little to actively dislike about it. Its consistent implementation of mechanics just barely glimpsed in the D&D Starter Set and Player's Handbook should give most players confidence that Dungeons & Dragons 5th Edition is going to provide interesting play and deliver on its core principles.
Bottom Line: A trove of fascinating creatures and fun reading, the Monster Manual makes D&D 5th feel like a complete game and game world.
Recommendation: Those who doubted the system based on the Player's Handbook alone may just be convinced by the Monster Manual's charms.
Designed by Chris Perkins, Mike Mearls, and Jeremy Crawford; additional development by Chris Sims, Rodney Thompson, Peter Lee, Robert J. Schwalb, Rodney Thompson, and Peter Lee. For Dungeons & Dragons 5th Edition. Published by Wizards of the Coast. Released September 2014. A copy was provided by Wizards of the Coast for review.