The cop show has been done so many times over. I was never much a huge fan of the genre in general, though I did like the occasional Hill Street Blues "realism play", etc., etc.
I think what I like about the series is that it doesn't paint everything in black or white, but really many layers of gray. Cops aren't always the good guys, and the drug dealers and punks aren't always the bad guys. Often there's a bit of good and bad in each, and often even in the same person. So it's those moral dilemmas and dichotomies that keep the show edgy, and the writing layers in some pretty intense police situations.
Personally, I find it one of the most compelling dramas on TV of the past few years. Not in a Breaking Bad way, though there are elements of that. The cast is quite good, but as with all shows that succeed like this it comes down to the writing (and some of the editing).