I've just read the first book, not the whole trilogy yet.
I was expecting it to be really dry, I don't know why. The last book I read by Asimov must have been when I was in middle school and I think it was above my reading level. Also, I had been told it was "focused on politics" and spanned several generations in its story so my closest frame of reference for what to expect was Dune.
It's actually more like a series of mostly self-contained short stories with a few decades from each one to the next, and the writing is very engaging. If you like Ursula Le Guin's Hainish cycle, Asimov seems to be interested in similar concepts with Foundation, albeit with worse politics.
I liked it a lot, but the things I didn't like are more interesting for me to write about so I'm going to just rip into it. Spoilers, I guess:
There's FTL travel, laser guns, and nuclear reactors you can fit in your pocket in this world, but they aren't super consequential. They're there, but not really explored. The real Sci Fi Thing that this book is about is Psychohistory: a field of mathematics that can be used to predict the trajectories of large populations of people and the various forces that move through them such as religion, economics, and nationalism. It's established at the beginning that it can be used to predict the actions of an individual, but it's not particularly suited to the task and requires a great deal of careful study of the person in question.
In the first chapter, The Smartest Most Geniusest Psychohistory Boy uses his big brain powers to predict the course of human history for the next several thousand years, and sets events in motion to minimize the period of time between the galactic empire collapsing and a new one taking form to just 1000 years.
Already I'm not really on board with the idea that a galactic empire is even a good thing to have. There's never really any challenge to that claim, and the book also seems to assume that it's so self-evidently desirable that it doesn't even need to try to convince the reader of it. Asimov doesn't pretend such an empire could sustain itself forever, but the time of "anarchy" (Asimov's word) between is filled with feudalism, tyranny, corruption, barbarianism, religious dogmatism, and anti-science thinking. The ideal setup is to keep those periods brief and the rule of the empires long. Maybe later in the series this outlook will be further interrogated and developed but for now I don't think there's any evidence there's supposed to be an unreliable narrator or anything, I just think Asimov's politics are bad.
Anyways, Psychohistory is supposed to work best on massive populations, right? Well, it turns out the rest of the book is some Great Man Theory bullshit where every chapter one guy singlehandedly changes the course of human history to the next crucial stage that Special Psychohistory Boy had planned out. So is this Psychohistory working on large populations or not? He certainly didn't have the opportunity to closely study any of these one guys, most of them weren't even born yet when he died. He predicts a couple events out to the exact day! Am I going insane? They keep talking throughout the book about how it needs to be driven by the "blind masses", but the actual plot is just a series of one guys being crucial to events unfolding the way they do.
I know I identified it as the big Sci Fi Thing in the series, which means you aren't supposed to ask too many questions because eventually the answer has to be "it just works". But when the Sci Fi Thing is FTL, you don't say that it works one way in chapter one and then have it actually work a different way for the rest of the book.
Last big problem: until the final chapter, there's no evidence there are any women at all in the entire galaxy, like, they're not even alluded to outside of a single joke in chapter one ("100000 men" "actually I think you're counting women and children as well"). Not a single one! And the one woman character is basically a clue about who's giving one faction nuclear weapons: she's been married over to their leader from another faction as a treaty. If she wasn't in the story we'd figure it out a few pages later anyway. I know "sci fi of that era was just like that" but seriously I've read fucking Conan the Barbarian novels with better written female characters. Still not great, but at least they "exist" and "have goals and motivations" which they "pursue and effect the course of the plot"
Finally, a tiny nitpick: throughout the book there are excerpts from the "Encyclopedia Galactica" which have a serious tone issue. They're just written in the same Isaac Asimov voice the rest of the book is written in! They're trying too hard to be dramatic and clever, and I just can't accept the idea that they're supposed to be encyclopedia articles. I just kept imagining how if they were on Wikipedia they would get WP:NPOVed in a heartbeat.