I visited Germany recently and enjoyed the graffiti tremendously. While there's some utter crap, I found the graffiti to be of significantly higher quality than what I typically encounter in my home country.
USian here, where graffiti is promptly removed, thankfully, because it's usually awful. But I think I know what you mean about Berlin, though I haven't been there personally. I have, however, been to Athens, Greece (around 2007). I swear, that city is just about the ideal environment for graffiti artists:
a. Nearly every building is made from concrete; my tour guide told me that there is a large concrete industry in Athens and that it is basically always the most economical building material.
b. Generally, plain concrete is irrepressibly bland, drab and awful to look at.
c. It's a "Mediterranean country", apparently; I asked a few of the locals, young and old, about the graffiti and their attitudes ranged from indifference to mild appreciation. Even the cops didn't seem to care - I asked one for directions once, and he encouraged me to check out some good graffiti a couple streets away from my destination. I was surprised at how relaxed the locals were about the whole thing.
Practically every wall of every building was screaming "I AM A BLANK CANVAS". Simple tags and anarchy signs were everywhere. Many city blocks had at least one elaborate spray-painted mural. Oddly, nobody seemed to spray-paint over one another's work; I guess there was just so much free space...
But at any rate, I took way more photos of the graffiti I encountered than of the Parthenon or any other famous landmarks. I'm glad I visited the place before all the riots started.