IMDB has it at #230 highest rated film of all time by IMDB users: http://www.imdb.com/title/tt1022603/Ā 8.1 out of 10 with 50K+ votes.
The Critics have it at 86% on Rotten: http://www.rottentomatoes.com/m/500_days_of_summer/
Consensus: A clever, offbeat romantic comedy, 500 Days of Summer is refreshingly honest and utterly charming.
#2 best-reviewed romantic film of 2009.
Then there are the Top 10 list of 2009 (to name a few):
About.com - 2
Associated Press - 7
BBC Radio - 3
The Capital Times - 2
Chicago Reader - 8
Entertainment Weekly - 6
Hollywood Reporter - 7
Miami Herald - 5
New York Daily News - 7
Premiere - 7
Richard Roeper - 4
Rolling Stone - 9
St. Louis Dispatch - 1
USA Today - 6
Then there are the awards:
Scott Neustadter and Michael H. Weber received numerous awards for their screenplay; including the 2009 Hollywood Film Festival's Hollywood Breakthrough Screenwriter Award on October 26, 2009,[49] the Satellite Award for Best Original Screenplay,[50] the Southeastern Film Critics Association Award for Best Original Screenplay (with the film also being named in the Top Ten Films of the Year),[51] as well as the Las Vegas Film Critics Society Award for Best Screenplay.[52]
Alan Edward Bell won the San Diego Film Critics Society Award for Best Editing,[53] as well as the film being named one of the ten best movies of the year by the National Board of Review Awards 2009.[48] The film also received two nominations at the 67th Golden Globe Awards announced on December 15th, 2009, for Best Picture (Comedy or Musical) and for Joseph Gordon-Levitt for Best Actor (Comedy or Musical). It has been nominated for four Independent Spirit Awards and received a nomination for the People's Choice Award.