It was pretty good until they left the suburbs (Nancy was still pretty awful, though, so your point stands). After that, it was like they tried to re-tool the show into a narconovella and failed hard.
Fair enough. I tried to think of one character in Weeds who was alright, and I guess it was just Isabella (a minor character that may as well have been a lamp in the background). I guess Heylia also had her head screwed on right, but she was also a minor character.
But yeah, I agree with that about irredeemable characters with the sole exception of "It's Always Sunny...". They're more caricatures of terrible people, IMO, so I guess that's what makes it work.