I just recently started looking at it, but I've found it very enjoyable. It has less "bull" than most other tv-series and movies tend to have, and a good story. Thumbs up.
Oh -- you have to watch every season!!
The characters have so much depth and it's great how the writers humanize even the worst elements in society. Even though I loved the previous seasons, the last season was really meaningful to me. Since I used to teach in the city (as did one of the show's creators), I totally identified with the storylines involving the teacher's relationship to the kids and the effects of outside factors on the student's lives. I really felt like the writers captured
exactly what happens in school system in real life and weaved it into a really powerful fictonal account.
Beyond the outstanding writing (I mean, how many other shows are out there that really make a gun-toting, gay, stick-up boy seem like a regular cat? Or that can really illuminate the struggle of a drug dealer - like Boadie - who's considering ratting out his supplier??) it's great to me that a lot of the characters are based on actual Baltimoreans and that well-known people in the city and/or state are acting on the show (did you catch the former governor of Maryland playing a security guard this season?)
That show is the only reason I get HBO. I seriously had HBO for the season and cancelled it a few hours after I watched the last episode. I can't wait for the final season. It's supposed to be about how the media affects the war on drugs in the city and since one of the other creators of the show used to write for the Sun, I'm thinking it's going to be great too

The sad thing is that the writers and the cast get passed over year after year for awards when, sorta like EY said, the show is probably one of the best shows on television - ever.