Saw Guardians of the Galaxy last weekend after missing The Winter Soldier in theaters, and telling my wife we had to see this one while it's still being ran. I really think it's the best Marvel Movie I've seen so far. It uses a very formulaic plot structure to give the movie a skeleton, but then paints that skeleton generously with humor, action, and a surprising few moments of emotional punch, which I didn't expect at all.
I'll agree with BlackNMild that Gamora was underused, but as disappointed I was in that, I was equally grateful they didn't turn her into a love-interest for Quill, and the story didn't bother with a love interest side-plot at all. I also like how they smacked you in the face with Thanos, giving you a few scenes with him in it, and also having the majority of the characters know him as a tyranical, menacing threat to the galaxy.
I'd really encourage anyone to go see it. Any quibbles about it I would have are minor, and are paved over by everything great about the movie.