It isn't without flaws, largely the result of its comic-book origins and decision to give it a 12A rating - most notably, we never really see the results of the frequent use of weapons and general violence; at one point the walking talking(?) tree Groot is nice to a child, later in the film we know logically that the child has to be dead, but we aren't shown it, or much else to point out that people are dying here, and in large numbers, at least in part because of the hero's actions.
Despite this weakness I enjoyed the film; the plot makes about as much sense as most comics, e.g. more than the average Star Trek / Star Wars movie, the sets and special effects are gorgeous and definitely inspired by the comics, and the characterization is pretty good, with running gag about the various aliens not understanding Earth metaphors ("But who put sticks up their asses? It sounds very cruel?"), Rocket Racoon's addiction to seriously overpowered weapons, and so forth. Incidentally, I discovered that I was scarily on the money with my characterisation of a Rocket-like Meerkat in a Star Wars game over the weekend; about the only difference was that Rocket never actually stepped out of a lift to assist two Jedi, leaving a pile of dead guards behind him, and said "Say hello to their leetle friend!" so I think I came out ahead on points. But Rocket has much bigger guns.
There are two short scenes in the end credits, neither very important; one shows Drax the Destroyer relaxing by sharpening his knives after the end of the story, with another character doing something funny behind him - the other shows the Collector, who also appeared in the end cuts of Thor 2, drinking with a surprise guest star.
There's apparently going to be a sequel in 2017, I look forward to it.