I have to admit, I have a bit of a love/hate relationship with gestures (fully supported in Storyboard 1.1 by the way!).
I love them when they work, often they are far more effective than alternative approaches.
What I dislike about them is that they are far from standardized so no two applications behave in exactly the same way, particularly between different operating system environments. The second difficulty is in the actual discovery of the gesture, what is intuitive to one user is not at all intuitive for the next and unlike keyboard shortcuts it isn't always easy to provide users with visual cuing.
Lukas Mathis has an interesting post on the topic of gestures, their discoverability and how they are similar in nature to command lines of old, well worth the read.