The synthetic stuff, we always called it fake weed, really doesn't have much withdrawal other than simply craving a high of some sort after quitting. Obviously if you experience something go to a Dr. and tell them the truth unfortunately most docs don't know much about this stuff as its new- really no one knows for sure about it.
Purely anecdotal: I know a lot of people who have smoked a lot of this stuff- a lot- and none have had physical withdrawal symptoms.