As you've likely heard, this is a fantastic movie. Its complex, constantly changing, interweaving storylines are not only incredibly fascinating to watch, but expertly written and superbly acted. This is intelligent storytelling, one that doesn't give all the answers to you, instead telling you just enough to let you figure it out for yourself. Basically, it's an incredibly cool movie. I could go on about the astounding cinematic talent on display here, the career-defining acting and so on, but I think you get the idea that this is the work of a master just by knowing it's a Scorsese piece.
Only one thing that bugged me: a few times in the first half of the movie, the sound editor toys with the music, sometimes reducing its volume suddenly and sometimes just cutting it off without warning as a character speaks. While I've seen both effects used effectively before, in this case it felt really off to me. That's pretty much it, though.
If films mean anything to you, then you have to watch this classic.
In a sentence:
The storytelling, writing and acting are prodigal, which makes this film's Best Picture Academy Award well deserved.