Well, I've made it through three of them, and I've seen worse movies. The first was the worst, there has been gradual improvement (but not enough), and I'm still at the "meh" level. But if you can't say anything nice, don't say anything at all, and I don't want to be silent, so here are a few of the things I liked about Vol. 3:
Dave Bautista. Or Drax, not sure, probably a combination of the two. He is my favorite character in these movies, in fact he's the only character I like on a consistent basis.
I am a fan of Elizabeth Debicki. I'll confess I don't remember her from Vol. 2 (I don't think she was in the first one), and her part is fairly small in Vol. 3. She is extremely tall, and on occasion you can see a film maker enjoying her presence ... here, it's when the King of All Bad Guys (played by Chukwudi Iwuji) comes up to yell at her about something, and he has to stand on a box to go head-to-head.
Karen Gillan is another very tall actress that I like, which leads to a barely-there trivia I told my wife: Karen Gillan is 5'11", and she's at least 4 inches shorter than Elizabeth Debicki.
James Gunn has a knack for picking timely music from the past in these movies. This time he works in Heart, Radiohead, Faith No More, the Beastie Boys, and the Replacements without going wrong, plus there is the inevitable return of Redbone, who were featured in the opening scene of the very first Guardians movie (it might still be the single best sequence in the trilogy):
On the other hand ... to point out the obvious, these movies keep getting longer (from 121 minutes to 137 to 150) to no purpose. The Raccoon has always been my least-favorite character, and he is featured in Vol. 3, so that's a negative in my book. My wife liked the story, and there is some effort to make us care about the Guardians. For the most part, it's not too sappy. But ultimately, I didn't give a shit, and I admit I resist being forced to care about a mutant raccoon.
So it's simple: you liked the first two Guardians movies, you'll like this one. If you didn't, don't bother with the third. Go watch Gunn's The Suicide Squad, which is actually good.