Official reply:
As of January 2011, ThreeTen is still what I'd classify as an Alpha. That means that I am
changing method names, classes appear and disappear etc. That said,
the code that is there is basically sound and tells the right time.
As such, I currently advise projects to use Joda-Time until ThreeTen
enters Beta, which will be this year (2011) at some point.
It is possible to have both Joda-Time and ThreeTen in the same
application. When ThreeTen is stable I intend to add the interfaces
from ThreeTen to Joda-Time providing interoperability. However,
interoperability will never be easy as the classes have the same
names.
Joda-Time will continue to be released for a good period after ThreeTen
is complete. But there will be a push to move to ThreeTen if JSR-310
completes such that it is in Java SE 8.
Update, March 2014,
As of 2014-03-05, JSR-310 was approved for inclusion in Java JDK 8. For more information, see the ThreeTen home page.