Other than knowing who Rocket and Groot are, my only previous experience in with the Guardians are some 1970's Star-Lord stories written by Doug Moench. I've got to say Star-Lord is pretty different in the movie, but I'm fine with that.
Guardians of the Galaxy is probably the most fun of all the Marvel movies. Through out the entire movie I didn't really care about the bad guys at all, but I had a ton of fun watching three of the five main heroes. I still don't really care about the big guy and the green chick.
Pretty much every scene with Star-Lord, Groot, or Rocket was great. As you can tell from the trailer, the film has a great sound track. I enjoyed the way the integrated the music into the story. I kind of wish they played Hooked on a Feeling during the end credits instead of the orchestral piece, just to send you home feeling good.
Stick around for the after credits scene.