For
It's Art like any other
Forces people to look and to think and have an opinion
Expression of ideas from people that may not be able to express otherwise
Brings people together (such as on graffiti boards)
Graffiti artists have rules and morals too, they don't just do it because it's illegal. (such as not to graffiti on churches, or graffiti over something that is better then yours etc)
Brings a more modern look to the city (some people actually pay for it to be done to their building, tastefully of course)
Often political meaning
Against
Vandalism
Ruins view
Hard on the eye
Personally I like graffiti when it is in good taste. In my home town in Liverpool there are a few graffiti boards with some really brilliant drawings on them. I think it is art like any other, many of them also have very political underlying ideas which are very interesting to think about. Just because it is graffiti doesn't mean it has to be dumb, or vandalising. It is an expression of ideas then can often, though not always, be a work of art.