Let's not mince words here. Guardians of the Galaxy is awesome, hilarious, super fun, and you should go see it.
[This review will contain ZERO spoilers, because spoilers are dumb.]
My disclaimer is that I'd never read the comics and was going into this completely blind. Although given that the title it's based on isn't particularly popular relative to the other Marvel blockbusters, there's a good chance you're coming from the place of blessed ignorance.
Whether you're devoted to the source material or not, the most important thing to know about Guardians of the Galaxy is that it's tonally perfect, riding the line between silliness and excellent action sequences as well as any summer fun movie in recent memory. It's exactly what a comic book movie should be. The bad guys are extra bad, the heroes are appropriately flawed. It's also much, much funnier than any of the other superhero movies of the modern era.
Super-brief plot outline: our hero, Peter Quill a.k.a. Star Lord, is a space thief. He steals a cool orb thing. Bad guys try to kill him for it, as does an otherwise perfectly pleasant green lady named Gamor. They're joined by bounty-hunters Rocket and Groot (a cybernetically enhanscd raccoon and a tree-creature, respectively), who are after Quill for some money. He's an in-demand kind of guy.
Quill, Gamor, Rocket, and Groot proceed to get arrested for fighting in public and are thrown into a truly terrifying prison where they meet Drax, a gigantic alien who is very good at breaking things. That's about as far in the plot as I can go without giving away spoilers, but suffice it to say that universe-saving mayhem ensues.
How about the cast? Chris Pratt as Peter Quill is super likeable, really fun and engaging, and, not surprisingly, very funny. Zoe Saldana as Gamora is super bad ass and oh my god please marry me. I don't care if our babies are green. Then there's former MMA and current WWE star Dave Batista as Drax, who has surprisingly good comic timing for such an impossibly huge and scary human being.
Groot and Rocket are voiced by Vin Diesel and Bradley Cooper, respectively, both of whom do wonders with a difficult job. I admit I was sceptical when I saw there was a talking raccoon and a human-like tree-thing in this movie, but I bought right into it, no problem. Lee Pace as Ronan (the Big Bad) is just the right amount of scary and mean. Also mean. Did I mention mean? Dude is hella mean. As for John C. Reilly as Corpsman Dey, well as always, Brule rules. The supporting cast and cameos are all quality, too. It's a very well-acted movie, which can't be easy when two of the main crew are completely CGI.
If I had one criticism to level at Guardians of the Galaxy it's that the exposition is sometimes so heavy-handed that you're thrown out of the moment. I get why they have to be there; this is a whole new world with its own rules, and the film-makers rightly don't want to slow down the story. Jamming it into one big run-on sentence is sometimes your only option. Luckily, these lines are at least effective, and and everything moves on so quickly once they're done that they only constitute minor speed bumps.
Meanwhile, the visual effects and special effects are both absolutely fantastic. You can't have a raccoon and a tree-thing as main characters if they look bad, especially if they're in nearly every scene. Not only do you believe Rocket and Groot every step of the way, you become very engaged with them as characters, which isn't easy to pull off. Also, there are a load of high-intensity spaceship fights and crazy battles and explosions, all of which look awesome. If you're coming for the eye-candy, you won't be disappointed.
Are there plot holes? Sure. Does it require suspension of disbelief? Only constantly. But Guardians of the Galaxy is one of those rare movies that's so fun you don't have time to care. This is the summer blockbuster we've been waiting for. Just go see it.