Went and saw it Thursday night (couldn't wait for Friday) as I'm a big fan of the Marvel movies and an avid comic book reader since I was a kid. Guardians was not a property I was familiar with until they began advertising and talking about the "next marvel movie" last year, so I had to bone up on it. I had high expectations for this movie based upon the treatment it appeared to receive in the trailers, which I was not let down by. As far as being true to the comics, there were some deviations, which worked well for the movie.
Drax is not as he is in the movie. In the comics he is not an ultra serious, thesaurus, but I thought that helped set him apart from the others in the film in a good way.
Yondu (the blue skinned Ravager played by Michael Rooker) was originally a member of the Guardians in their first incarnation before the current team took up the mantle. He was actually a good guy, not a space pirate/marauder/mercenary.
Ronan the Accuser wasn't a fully fleshed out character, and is actually pretty badass as basically a cosmic judge of judges.
The cosmonaut dog was actually Cosmo, the head of security of Knowhere in the comics, an actual character that was reduced to a visual joke, which is fine for the movie storyline.
There were also quite a few missing members of the Guardians but for obvious reasons they had to cut down the amount of cast.
NERD RANT CONCLUDED
Anyways, good movie on the whole. I wanted more Thanos and am eagerly anticipating him becoming the future villain that the Avengers (and Guardians) will deal with come the next installments of Avengers 3 and Guardians 2.