Well i think graffiti is artistic, but as Andrew said (somehow) i find that if its well manipulated it can give a great impact and be an artistic intervention specially not overlaying another artists work.
I used graffity in my FYP you can check it out on my page, i designed a wall only for graffiti in front of a skate park, and i did that because graffiti was all over the city i worked on. I found out that by designing a space, a wall for this kind of intervention makes it more valuable and improves it esthetically. Gangs or whatever, people should get the chance to express themselves and graffiti nowadays has become a nice way to do it… just give em space