|Here there be monsters...|
WHAT YOU SHOULD KNOW: Retail price is about $50, and the book is smidge over 350 pages. It provides the monsters for your D&D game, though you can check out the free DM Guide to get a selection of monsters, and all the stat blocks you need for Tyranny of Dragons are either in the respective adventure books or the free supplements.
So really, you have a lot to work with, already...but you can download and read those for free. The Monster Manual costs money, so I'm going to talk about that.
The production values are fantastic, as should be expected. I like the art direction for Fifth Edition, for the most part, and several of the art pieces look fantastic and evocative, like the Slaads (I love the Death Slaad), the vampire layer, most of the dragons...special props go to The Death Knight which, if it isn't Lord Soth, it certainly evokes him, and the Werebear, which spells out the difference between a bear and a werebear by brandishing a battle axe.
There's not a ton of surprises here for the D&D faithful: Orcs, goblins, hobgoblins, bugbears, chromatic and metallic dragons, drow, mind flayers, beholders, nightmares, demons, devils, the various giants...even The Tarrasque. Honestly, I'm not sure there's anything I expected to be here that's missing. A couple of things have been renamed (Titans are not Empyreans, for instance), and some (like Drow and orcs) get multiple statblocks to show off the variances within the race.
Stat blocks are a bit more streamlined in some ways, one of the most notable being that creatures with reams of spell-like abilities have been greatly reduced (this may have happened in 4e, I honestly do not know)...with dragons, demons and devils standing out to me as having reduced (or missing altogether) spellcasting capabilities. Monsters do keep Ability Scores, however, something I approve of greatly.
The bigger and nastier monsters get Legendary actions, which often give them extra attacks after other characters have gone, and some get Lair Actions, which make them even deadlier if you fight them on their own turf (lookin' at you, dragons). Fight a red dragon in its lair and it may start a spontaneous earthquake, or take on a lich and find that it can summon the spirits of the those that passed in its lair and use them to tear away at a target. Another awesome touch is that creatures with lairs actually impose their will on the surrounding terrain, twisting it to reflect their temperament (like creepy fog that grip the land when a vampire takes a lair, or people within a mile of a copper dragon's lair becoming prone to fits of giggling).
The monster entries typically provide about three plot seeds/lore tidbits per entry (sometimes more, sometimes less, but that's probably a fair average), though some (like liches) get more, but the Monster Manual infamously omitted indexes by environment or challenge rating, as well as the guidelines for actually creating monsters (all of which were found in the DMG).
It's a good book but not strictly necessary, especially if you are running the Tyranny of Dragons pretty much as written, or if you are comfortable looking at the free monsters and extrapolating from there, and while I do enjoy the book, for sheer utility the 2nd Edition Monstrous Manual trumps it, in my opinion.
SIX POINT SUMMARY:
1) The 5th edition Monster Manual is not a necessary purchase if you are following the published modules, and I suspect this will remain true when the next arc begins.
2) Lair Actions and Legendary Actions are a great addition to the game, but the application is odd in one or two places. For instance, by the included flavor text, hags SCREAM for Lair Actions and Regional Effects, but they sadly did not get them.
3) The restrictions on just who among the monsters is getting spellcasting abilities makes it seem more important when it does happen...and Legendary Actions and Lair Actions are a much more effective tool for making red dragons absolutely terrifying than giving him 3rd level spells. Although this makes the whole Chromatic Half-Dragon (which are included in the book) thing even more awkward now, since Chromatics still don't shapeshift and now can't polyform, either.
4) Some great sidebar variants help you tweak some of the monsters even further, getting a little more bang out of them without taking up whole pages or statblocks on them. Along those same lines, Dragons are now divided into four age groups instead of twelve, which just seems much more user friendly to me.
5) Not only did the Monster Manual exclude the index by environment, but it left the environment out of the stat block for each monster, an omission that's more than a bit bothersome.
6) While the lore pieces do a good job of helping the DM bring the monsters alive, the beginning of the book absolutely stresses that nothing here should be taken as law, that exceptions exist to every entry and that it is absolutely your world and these entries are just to have provide inspiration. Perhaps more importantly, it provided the 5e rules baseline for all the old familiar faces to use as tweaking points for creating and converting your own monsters.
I personally think it's a great book, but not quite the complete homerun that was the Player's Handbook, but I certainly still came away confident in the development team's grasp on 5th Edition's quality.