I've seen some really good "artwork" graffiti... some of those guys have real talent.
Not that it makes it right, I'd be mighty pissed off if someone painted my property without my permission.
I think the compulsive taggers are far far worse as they seem to do 2sec tags over every inch of every space they ever visit... and it's horrible to look at. I think the micro-tags are mostly from bored kids...
I'll have to dig around in my photos to see if I can find some of the "good" ones I've found/photographed in the past...