Well I can see why this is the biggest-selling movie of all time (by a large margin): in 1939, this must have been a hell of an event - a phenomenon, even. I couldn't believe that even 72 years later, some of the images were so stunningly beautifully crafted that they actually took my breath away.
However, I can't really say the same for the story. While the first half (pre-intermission) stays quite interesting throughout by interweaving the romance through the Civil War storylines, the second half quickly descends into Soap Opera whining by putting the love rectangle (?) at the forefront. The film's inhumanely long 4-hour running time doesn't help, although it is successful in making the movie feel like a sprawling epic.
Even worse is how completely moronic and detestable the main character is. The whole movie's story somewhat maddeningly hinges on her emotional stupidity; her love for the weak, bland, married Ashley. The whole last hour is in fact devoted to a back-and-forth caused entirely by her sentimental incompetence. As a result, the second half quickly loses steam, only to be saved by the glorious "Frankly, my dear..." line at the very end. It very much feels like filler.
So then, while it is undeniably a cinematic classic when seen in context, it's only really worth it for the excellent first half - anything beyond that and you're only really in it to be able to say "I've seen Gone With The Wind!"