There is a strong, strong case to be made (in my opinion, of course) that the reason history permitted the rise of Marvel Studios was to ensure the existence of the Guardians of the Galaxy movies. Yes, that means I liked this movie. Full disclosure, straight out of the gate.
I'm going to assume you've seen the original Guardians of the Galaxy. If you haven't, you may want to skip this review, as it will contain spoilers for that movie. I'll avoid spoiling GotG2, however. Spoilers, here, are defined as any information you wouldn't be given in an official trailer for the film.
GotG2 picks up pretty much where the first one left off. The team is as we left it, and they're off doing exactly what Star-Lord said they'd do at the end of the first: "Something good? Something bad? Bit of both? Bit of both." The opening sequence, though, is when I knew I'd enjoy this one too. The opening battle is used as the credits sequence - but the battle itself is fully a backdrop for what's happening. I won't tell you how, but I will tell you it charmed the heck out of me.
As the trailers promise, we learn (a LOT) more about Peter's dad in this movie. He in fact drives one of the movie's main plots. Which leads me to one possible negative comment - this movie has started suffering from the same problem that the mainline Avengers movies began to stagger under, namely that the storylines in it have begun to proliferate in order to maintain continuity with the larger MCU plan/story. So far, in GotG2, this doesn't sink the film, but it definitely changes the tone a bit. This movie isn't about The Guardians alone, it's about the Guardians and how they as a group (and as individuals) fit into Marvel's Master Plan(tm). Remember, though, that said Master Plan is a comic book universe story - in other words, written in a frenzied deadline manner by a group of professional pubescents.
What saves GotG2 is that some of its other themes and subplots are good enough on their own to hold up the weight of the web of storyline cables that Marvel has draped over it. The notion of family is the biggest, I'd say. It's addressed in a number of ways, both ham-handed and not-so-ham-handed (I won't go so far as to say subtly, though) and some of those bits are some of the most rewarding to watch.
The other thing that both makes the movie great and weighs it down? Fan service. There is practically an infinity (har har) of references in this thing, and while some of them are to mainstream things or things in the previous movies, a number of them are crafted specifically for the 1980s/1990s Marvel comic nerd. I won't even begin to go into what they are, because for one thing I'd never do them justice (not being one of those types) and for another there are a plethora of YouTube videos gleefully pointing them all out; if you care, look those up. As an example, there isn't just an after-credits scene. There are five of them spread throughout the credits sequence.
So yeah. With all that, it's amazing the movie comes out as well as it does. But it really does. I mean, when a movie makes an explicit reference to Galaga in how it CGIs its space battles (no, I'm not kidding, and damn it, I didn't mean to get into the references) you know it a) doesn't take itself too seriously at all and b) is talking specifically to a particular group of audience members. Those people that were kids in the 1980s and 1990s, like Peter Quill; those people who remember big chunks of that time and its U.S. consumer culture fondly. If you're one of us, you'll grin most of the way through this.
The characters evolve here, too. More so than in most comic book movies. Maybe not subtly, maybe not in particularly inventive ways, but enough to free them from the stultifying stasis that most comic book characterizations (when they even exist past a tl;dr origin story sentence) can't approach.
The soundtrack? Awesome Mix Vol. 2 delivers, here. Excellent use of Fleetwood Mac, yo. The comedy? In full effect; what GotG promised, this one continues to deliver.
In short, go see it if you liked the first one. If you didn't like the first one, maybe it's not for you. But maybe it is, too.
As the closing teaser states in a 1980s-tastic laser scrawl: "THE GUARDIANS OF THE GALAXY...WILL RETURN."