I know relatively little about them, but they appear to have a high potential to be a clear language enhancement.
IIUC, they allow the running thread to be suspended, while another operation occurs. The typical use case is to allow a server to send a response to a browser and wait for the next request. This model reverses the standard server side code, making it easier to understand.
It fits well as a Java enhancement as it is
1) Difficult to do as open source using existing JVM features
2) Provides a real improvement in clarity as to what the goal of your code is
I am sure that there are others on this forum who know more, but this is something I would definitely like investigated for 1.6.