A lot of people consider this to be one of the greatest movies ever made, and I can kind of understand why they'd think that. Unfortunately, I only thought this movie was above average. The main interest here would be that it's an incredibly in-depth and complete (and super-long) look at the gangster lifestyle, coupled with great acting (I thought Ray Liotta was incredibly annoying, though) and great writing. If that sounds amazing to you, then you will love this movie. I, on the other hand, have never really found gangster movies to be particularly interesting. They're unique and show a facet of humanity that we rarely see, sure... but they don't resonate with me in any way as I can't associate myself with any of it. Basically, I can't get emotionally invested in a movie like this, which turns down my enjoyment level.
Still, it's somewhat fascinating to see how everything evolves over three decades (50s, 60s and 70s, no less) and think that this is heavily influenced by a true story. And I couldn't disagree that this is definitely a great addition to your movie culture.
In a sentence:
Extremely complete, well-acted and well-written, but emotionally non-resonant.