Read on, safe from spoilers!
My only real criticism is that some of the character development is heavy handed. The intended theme seems to be that you don't always get on with those you love, and I could have got that on my own without it being stated in dialogue, let alone stated multiple times. There are a couple of occasions where characters drop everything to talk about their feelings and again while it's not bad as such, it is a bit clumsy.
(The film is quite sweary too, much more than I'd expect given its rating, and there are a couple of willy jokes. That's not a problem for me, but bear it in mind if you're going to take kids.)
Other than that, it's all gold. The central characters and their performances are as good as before, except this time Gamora gets something to do apart from looking pretty. Baby Groot is adorable, and Mantis even more so. Kurt Russell -- that's not a spoiler; he's in the very first shot -- is as wonderful as Kurt Russell always is, although I was disappointed that he didn't at any point wear an eye patch.
The plot isn't complex but there are enough moving parts to keep things interesting. There are multiple factions roaming about, getting in each other's way, and the main antagonist is compelling; they are not an outright villain, just someone who made the wrong choice in the past, and that gives them a bit of weight. It's one place where James Gunn doesn't stray into overwriting his characters' motivations; other writers -- (cough) George Lucas (cough) -- would have wrung every bit of melodrama out of the villain agonising over their choice, but Gunn just gets on with it, and it works well.
Just as the first film was, the second is funny, more overtly comedic than the rest of the Marvel oeuvre, and most of the jokes land. Drax and Mantis get most of the best lines, but there's also a nice extended routine about a character's name, and some good visual gags scattered through the film.
The film looks good, with bright, colourful, and varied visuals, maybe even more so than the first. Perhaps there are a few too many characters wearing some form of muted leather jacket but that aside it's never dull to look at. Music isn't used quite as well as in the first film, but there are a couple of superb sequences; the opening credits are joyous and if you don't break out into a big stupid grin during them, then you are dead inside. That bit is up there with The Lego Batman Movie and Indiana Jones and the Temple of Doom in my book.
(I don't have a book.)
On the subject of credits, there are loads of mid-and-post-credits scenes in GotG2 so if you're into that sort of thing, stay right to the end. I'd say only one of them is "relevant" but they're all good fun.
I love the first GotG; it's a big, bold, colourful space adventure, a pitch perfect adaptation of a Saturday morning cartoon we never had. I didn't think they'd be able to capture that magic again so I was worried going into the cinema, but my worry was unfounded. The sometimes clumsy writing is a bit of a disappointment but otherwise Guardians of the Galaxy Vol 2 is another triumph for Marvel.
If you do go and see it, please consider donating a little to help the creator of Rocket Raccoon pay for ongoing medical care.