It's hard to explain just what makes BECK such a brilliant read, after all, it is "slice of life" so there are no obvious story hooks or big action pay offs.
I like following the characters. They are consistent underdogs, always fighting an uphill battle to succeed. It's a pretty inspirational story. The author obviously has a great passion for music and it shows in his artwork, there is strong sense of authenticity and punkish attitude in the way things work out for Koyuki.
The musical scenes themselves are great, I was utterly captivated the first time I read the summer music festival performance. Of course, you have to imagine how it sounds, but you are able to take a lot from both the ultra stylish poses during the band performances (they look lost in the music) and the sometimes mystified audience reactions.
Suffice to say, I recommend to look at reading BECK. It's the best manga series that I've read.