As the reviews come in, Rotten Tomatoes collects the reviews and gives a freshness rating. With the world premier of Guardians of the Galaxy having taken place, reviews are coming in and as of right now there are 19 files (3 from their "top critics") and the movie has a 100% Freshness rating. Meaning that all 19 reviews have been positive. The movie is also carrying a 99% audience wants to see rating.
For a frame of reference, current movie ratings Dawn of the Planet of the Apes is at 91% fresh while Sex Tape is at 19%… not so fresh.