This ISN'T as great as Chapter One
In fact, it's only good. And I never, for one moment, thought it would be better than Chapter One.
I think it stems from the adult portion of the story, which was never going to be as captivating or interesting as the child part. That shows in the book, which "interlaces" the two time periods together, sometimes even switching between them in one moment, and I'm thinking that Stephen King wrote it this way to maintain interest in the story. So you're reading along, enjoying a child part, and then it flashes forward in time to an adult part and it's not so good. This unusual structure was mostly used in both TV and movie adaptations (1990 and 2017/9) and they both suffer for focusing on the adults in the second half.
And why should readers and viewers care about what happens with the adults? There is something positively engaging about a group of imaginative children facing off against an ancient cosmic evil in the form of a clown and defeating it through loss of fear. Understandably, they can't kill it, yet. But having them come back as adults and facing off against it again, with the physical and mental strength necessary to kill it for good, but without the innocence of childhood, is not so compelling. This has always been a problem with the King book, but he knew this and focused on the children more than the adults.
This movie has other problems for me, namely the fact that the original cut is FOUR HOURS and there are, somehow, gaps in the story, and yet the main criticism is the length. Even the trailer features a couple of moments that are not in the movie, but are actually deleted scenes. Then there is the de-ageing of the children from the first part, which is inconsistent at best. It's like parts of their bodies have aged more than others, it looks strange sometimes.
I wasn't bothered by the hunt for the childhood objects, nor the Native American stuff, as it was implied in the book. And Pennywise would've tormented the Losers more than we saw in Chapter One: Stan implied it with the line "and the woman I keep seeing", meaning that the Painted Lady tormented him several times more than Chapter One showed, and thus no doubt the others, too - Chapter Two simply shows us these events. The CGI for some of them is odd, but then the budget was tight, anyway. $70m? Sounds a bit tight to me. As a result, some of the monsters will look cartoonish, but then that's most likely Pennywise's imagination. One scene I didn't really like was in the climax and they're forcing Pennywise to change forms constantly, and he sort of "wobbles" between forms like tuning a TV, which didn't really happen in Chapter One, he just changed instantly.
The casting is great, and the adult Losers act just like their child counterparts, but with the change that comes with the passing of time. I even liked the idea that they were "infected" with IT's damaging power, and that they would eventually kill themselves before they were 70, so they HAD to defeat it then. They already lost one (Stan).
But really, this adaptation of IT has always been about the Losers and their bonds of friendship, and overcoming your fears and defeating evil. I love those parts of the story most. As people have said about Chapter One, it's a coming-of-age story but with a killer clown, and this continues it. The ending is bittersweet, but uplifting.
IT Chapter One: 9/10
IT Chapter Two: 7/10