As a huge fantasy fan, and someone who has read the first six (!!!) Malazan novels, I would say: they’re not worth it.
What starts out kind of fascinatingly bizarre and ornate and feeling like it might have some real depth to it ends up being a shallow festival of Gary Stus and Mary Sues (and I don’t use those terms lightly, unlike a lot of people) doing random shit “because they’re awesome”, with absolutely no insight whatsoever into the human condition, nothing intelligent or coherent to say philosophically, and with terrible leaden prose that is sometimes so bad it’s literally hard to understand. Plus tons of sexual violence for no particular reason, including some from characters we’re meant to sympathize with.
You’d get more out of reading a really good tabletop RPG sourcebook.
Basically there’s one good idea in Malazan - that of a fantasy setting with a history that goes back not a few hundred years, or a few thousand years, but hundreds of thousands of years. What might that look like? What might attempts at immortality and surviving the ages look like with hundreds of thousands of years in play? And it’s based on a tabletop campaign the author played/ran over a couple of decades, in both AD&D 1E and GURPS Fantasy (mostly the latter, and it shows if you know GURPS Fantasy).
The first book isn’t very good, but the next two are okay, maybe even good, but then it’s just downhill from there.
Major problems I’d list:
Full-on Grimdark. Like, you think Joe Abercrombie is grimdark, say? Compared to Malazan his books are a walk through a field of daisies.
Few likeable characters, and they tend to get killed/sidelined. Quick Ben and the Bridgeburners seem charming initially but they get sidelined or killed off, or turn into twats themselves.
Total inability for the author to “kill his darlings”. It’s very clear which characters in the novels were the “player characters” in the campaign (or DMPCs), because they’re all protected by 50 foot of solid steel “plot armour”, and ridiculously overpowered and the plots bends itself to give them opportunities to look cool or do what they want, even if it makes no sense, and doesn’t like, mean anything. And most of them are fucking insufferable twerps, either maudlin Drizzt rip-offs, or sulky Elric rip-offs, or incredibly full-of-themselves jerks who people are inexplicably impressed with.
When we escape these characters, the books tend to improve a bit - but it’s never for long.
Endless suffering-porn and sexual violence, especially as the series goes on. Any characters who aren’t plot-armoured go through horrific suffering, for no real reason other than to show what shitsack world they live in and how powerless they are next to the badasses.
Completely inconsistent tone and style, and poor differentiation of character “voices”. This is straight-up the result of being a bad writer. Aside from Yoda-type speech patterns, characters largely all speak the same way, think basically the same way, and so on, which is bad, but what’s much, much worse is the totally inconsistent tone and style of the writing, and the author’s inability to convey tone. There’s a whole subsection in book 5 or 6, which apparently supposed to be “funny”, except it isn’t funny, and nothing clues you in that it should be funny (and this even most serious fans of Erikson are surprised to learn its intended to be funny), and not only that, it’s trying to be funny about something the books have at length laid out isn’t the case (specifically that things from the past are often outdated, rather than badass). And this subplot goes on and on and on. Other stuff is supposed to be taken seriously, but it’s hard to do, because so po-faced it is accidentally funny. Other stuff still gets all hazy and like bad, ugly poetry, and you’re not even sure what’s going on, or if it’s a dream sequence or whatever.
The books are constantly adding new magical stuff to the point where you can’t even tell if something is actually real or imaginary until it’s been going on for some time (especially as character do hallucinate and dream a lot as well). This isn’t one of those series where the magic is consistent and makes sense. This is one where totally wild new things are continually added and you have no way to assess how actually powerful or important they are, and even what the characters say about them is often misleading. This leads to some weird anticlimaxes.
Nothing meaningful or insightful to say about human nature/behaviour, no philosophical ideas that aren’t both shit and contradicted immediately and no real connection to humanity. The author seems to have little empathy for his characters, and to just enjoy badassery above all else, even if it’s entirely pointless. This makes the books seem extremely cold. What’s really sad is, there are a few relatable characters, but they’re in this sea of endless magical assholes, and the author isn’t very interested in them.
There are 10 books and they’re pretty much all fucking gigantic. And whilst you may have a few good ideas for an RPG campaign from reading them (most of which you will get from books 2 and 3), that’s all you’ll get from them.
TLDR: Malazan is exactly what people who hate Fantasy imagine all Fantasy series are like. A massive collection of unmemorable, unlikeable characters, with ridiculous names, and ludicrous backstories, engaging in meaningless magical ultra-violence for thousands and thousands of pages, interspersed with exposition thick enough to suffocate you.
If you want something that is some ways similar, but not rubbish and endless, you might try Brian Staveley’s trilogy which starts with The Emperor’s Blades (leave the prequel Skullsworn well alone though, terrible). If you want other recommendations which are actually outright good:
NK Jemisin - trilogy that starts with The Fifth Season
Naomi Novik - Uprooted
Sabaa Tahir - An Ember in the Ashes (YA and has some YA tropes but works really well, avoids a lot of pitfalls and is a fun read - four book series IIRC).
Leigh Bardugo - Six of Crows/Crooked Kingdom (also YA, but also very good, free of virtually all YA tropes and better than most fantasy)
NB I don’t necessarily recommend other books by the same authors - Novik and Bardugo are pretty variable (or rather, Bardugo has steadily improved, but her earlier stuff is pretty weak).
I’ve just started on The Thousand Names by the ridiculously-named Django Wexler (right up there with Adrian Tchaikovsky for pen names), which is good so far in that it’s a good-natured, suspenseful sort of fantasy take on Sharpe (or at least that technological era). I’ll report back if it stays good.
Also if you aren’t married to conventional fantasy and just want to read something amazing I can’t recommend Gideon the Ninth by Tamsyn Muir enough. I was a bit dubious at the start but it is awesome (and is arguably about six different genres, in a good way).