On a purely technical level, it is flat out the greatest comic book ever - certainly on the American side of things. The more you understand the language of comics, the more you'll get out of it.
It's also just a damn good read in terms of story and characterization. Dave Gibbons' art is also pretty underrated.
That said, there is a coldness to it that may well put off some readers and it's why, though it is up there, it isn't at the top of the list of my all-time favourite comics.