See, that is what i found to be the weaker side of it. The storylines. It revolved around a formula a bit.
Took me a while to get into S1. Because it revolved around the strength of characters more than the strength of the story, which built slowly but was almost secondary, and it took time to get to know the characters.
S2 then was almost the identical formula, but with white immigrant factions as opposed to baltimore street gangs.
S3 then went back to the more familiar characters (which in fairness were kept in it through s2) but the greeks/poles were never to be heard of again.
S4 introduced a whole new generation, JUST when you were getting into some of the routine, so it actually threw.
me off a fair bit.
S5 became more about the inner police and electoral departments.
The strength of it for me was definately the characters (and the cast who pilled them off who don't always get that credit), rather than the storyline, is my point.