I almost always buy the extended version of movies available. Badder Santa, The Hangover, The Last Emperor, and Lord of the Rings are among the many films that have significantly longer footage than the theatrical release.
Often, I find the original versions to be better. Apocalypse Now, Blade Runner, Payback were all better in their earlier cuts, though the extended or reworked edits were compelling. (I have 4 or 5 different versions of Apocalypse Now).
I am about to go buy Donny Darko, but I am not sure which version to get, and would like input on that as well.

Topics: film, movies, cinema, directors cut, donny darko