I got my start in roleplaying with AD&D 2nd Edition. I can't say that I read those books especially closely (mainly because I doubt 14-year-old me read anything closely by my current standards), but I did read them repeatedly. So my experience of reading AD&D is odd - it's like looking at something familiar in a funhouse mirror. Everything's a bit off. A bit shabbier, less polished, but still recognizably the same.
I can't say that I like the AD&D Players Handbook. It's just filled with these inconsistencies and questionable choices. Like spell ranges being measured in inches, to represent space on a tabletop battle-mat, but then the inches themselves having contextual translations based on the conditions of the game. If your characters are outdoors 1 inch equals 10 yards. If they're indoors 1 inch equals 10 feet. But that's only for spell ranges. Areas of effect use the inch = 10 feet translation regardless of where the spells were cast. It's needlessly confusing, even before you count in the fact that sometimes the text slips up and uses true measurements in the descriptions. Like, seriously, what is "Darkness 15' Radius" even supposed to mean?
It feels weird to say this, but BECM D&D was a more sophisticated and polished game. Perhaps not surprising, given its later release, but still, I thought the "advanced" in the title signaled something. There is some added complexity, but I'm not sure it adds all that much. High attribute levels have a larger variety of effects, but in practice a simple modifier does almost everything that you'd want with much less fuss. And decoupling race and class theoretically offers more diverse character options, but in practice only the halfling thief was better than its BECM counterpart.
I was interested to see what was cut from AD&D 2nd edition - a couple of spells, the half-orc race, and the assassin class. I'm guessing they wanted to get rid of the evil options as the game became more popular. I'd bemoan the loss of prime anti-hero material, but honestly, AD&D doesn't do well with the concept. Alignments are very non-porous. Good is good and evil is evil. There's very little room for nuance, like a character who is kind towards his friends, but ruthless towards his enemies, or someone who is generally good, but pragmatic enough to use poison. The game seems to work as well as it does because its basic mode of play is utterly mercenary - go into trap-ridden underground chambers and steal treasure, possibly fighting monsters along the way, but ideally avoiding them if possible. It's all about the loot and scoot. And in such an endeavor, good and evil can work side-by-side.
Ultimately, this book is highly flawed and I don't think it has enough of an upside to overcome those flaws. Dungeons and Dragons, taken as a whole, with all of its myriad expansions and spinoffs, is fertile enough with brilliant ideas that this book is worth it for the foundation it provides, but it cannot stand alone. Hell, even to the extent that you might like old-school D&D, this book can't stand alone. It lacks all of the basic rules for combat and exploration and treasures. Those will be in the Dungeon Master's Guide, which apparently wasn't yet released at the time. It's odd to think that there was a period of time where Dungeons and Dragons was literally unplayable, but I guess it was backwards compatible. The players could work from the advanced book while the DM used the original book, and apparently that was functional, but in retrospect it could potentially be a trap for people looking to get back into old-school roleplaying.
UKSS Contribution: This game does not have the same out-of-control weirdness as BECM D&D, but that may be because it doesn't focus on setting elements. Most of its unique character comes from its spell list (roughly half the book, grr). The most interesting thing is the Clone spell. The mage creates a duplicate of the target with all of its memories and abilities, but the clone and the original cannot abide the existence of the other, potentially going insane if they persist too long.
Dungeons and Dragons resolutely avoids allowing its magic to greatly affect the setting, but I think it would be fun to explore the implications of this. Maybe tone down the forced insanity (AD&D, for all its "rulings not rules" philosophy, doesn't shy away of enforcing genre cliches in its spell descriptions) but play up the paranoia, existential angst, and transhuman decadence that would inevitably result from this power being in the hands of the rich and well-connected.