I love graff art! Back in my teens I used to do bubble letters on whatever surface I could find. As I got older, I went from bubble letters to images then stopped all together in favor of writing rhymes. But I never stopped loving the craft. What I like to see in good pieces of graff work is a strong technique. Meaning if you're doing letters, the letters have good borders, colors and are easy to read. If you're combining text with images the same rules apply accept the image has to be creatively done to compliment whatever the text says.
If you love graffiti art check out these posts (Local Graff Art and Local Graff Art 2) by clicking on the titles. The series is ongoing and features graff from different artists in Holland, so check back periodically for updates.