Graffiti's not my favorite thing ever -- it far too often results in the destruction of private property.  Much of the older parts of European cities, for instance, are significantly marred by the heavy graffiti which covers their historic buildings.  Venice, in my opinion, is particularly damaged. 

However, there are times when graffiti is a good thing, when it can be an act of peaceful resistance to unjust behavior of an oppressive state.  The Israeli West Bank Barrier is only a few years old, but its similarities to the oppression of the Berlin Wall are clear.  It has been denounced by human rights groups worldwide, and though it is certainly understandable that Israelis seek protection from Palestinian terror attacks, this wall has destroyed the property of innocent Palestinians (including important farm land) and is without question a ridiculous abuse of power. 

The graffiti which has appeared on the wall is not only reflective of peaceful protest, but often beautiful as well:

See more pictures (with zoom!) here.