Reading Monster Manual II was a strange experience, because it's largely a better book than the original Monster Manual, but it greatly suffers from the fact that most of the really iconic monsters have all been used up, presumably due to the first book's insistence on having 500 monsters in 300 pages. This book has half as many monsters in 2/3rds as many pages and while it's still not entirely where I want it to be, in terms of the relationship between fluff and mechanics, it did generally feel better to read (when it wasn't repeating the long, dry rules for "improved grab" and "swallow whole" about 50-100 times).
And while it didn't have any truly iconic monsters, it did have a few almost iconic ones (I'm defining "iconic" here as "could Hasbro profitably release this creature as a plush"). You've got the Neogi and Thri-Kreen, gem dragons and Galeb Duhr, Myconids and Grell. The cast has got some charisma.
Unfortunately, it feels like the bulk of this book's charm comes from previous edition holdovers. It's hard to say with certainty, because even by this point D&D was swamped by decades of old canon, but I'm going to do an internet search for the first appearance of a few of this book's best monsters (whose origins I don't immediately recognize):
Yak-Folk - Land of Fate boxed set.
Clockwork Horror - Monstrous Compendium Spelljammer Appendix 1.
Glimmerskin - original to this book.
Corpse Gatherer - original to this book.
Which is actually a better-than-expected ratio, if I'm being honest. And I know that's kind of a shitty take for me to have ("the identifiably old stuff is noticeably better"), but it's not really an assessment of the book's craft so much as an observation about the power of the nostalgia filter. The brand-new stuff is competing against things that were hand-picked from a previous edition as notable stand-outs, so of course if you compare the hit-or-miss process of creating new monsters to the guaranteed hit process of just picking the known hits, that's not going to be a favorable match-up. However, I do think that it's an indictment of the edition's editorial choice to have long stat-blocks accompanied by short flavor descriptions. There's no reason at all that the Abeils (bee-folk) couldn't have been as popular a Lawful-neutral foe as the Formians (ant-centaurs), if only they'd gotten a Planescape-style lore dump.
All that being said, this book was mostly an enjoyable read, despite the fact that the more 3e-style stat-blocks I see, the more each new one feels like a unique brand of torture (I am serious about that "improved grab" thing. I was getting sarcastic about it towards the end, "oh, is this creature going to get the ability to maintain a grapple with a -20 penalty, how original"). But that may just be me refusing to bear the cognitive load of decoding them. In theory, a Gravecrawler's Combat Reflexes and Mobility feats are going to be an important part of its combat repertoire, but am I going to remember to use them?
The biggest weakness of this book is not, however, its monster-statting methodology, mechanics-lore balance, or overall monster curation. Its biggest weakness is more of a branding issue. I'm not sure I'd ever remember that this book exists. There are useful creatures in here, but if I'm building an adventure or plotting out a game, I'm going to reach for the first Monster Manual, and if I don't find what I'm looking for, I'll probably just adapt something, rather than reach for a second volume. I think a more focused monster supplement, that just gave me creatures from a particular location (like the outer planes) or with a particular theme (like all undead creatures), would greatly aid discoverability.
Although, even as I say this, I realize that's maybe too much to put on this book. In a way, Monster Manual II's haphazard creation is simply an artifact of the first Monster Manual being assembled in exactly the same way. And I can't really blame the first Monster Manual for doing it that way, because it's a core book, with a mission to serve a wide variety of potential games. The origin of this fault was simply the naive (or possibly hubristic) notion that you could just slap a roman numeral on a core book's title and make the new volume core as well (this is one of the few faults I'm willing to concede about my beloved 4th edition).
Overall, my assessment of this book is "why not?" I've said it before, and I'm sure I'll say it again - monster books are nearly impossible to screw up, having a more-or-less perfect fantasy format, and Monster Manual II is no exception to the trend.
Ukss Contribution: This one's a little bit cheating, because the thing I'm picking was likely a runner-up for the 2nd edition Monstrous Manual, but what can I say, I kind of love these little guys - the Myconids, sentient mushroom people who are peaceful by nature, but if you attack them, their defense is to get you high as fuck.