Well, the short answer is I have absolutely no idea. I have never used one in anger but have played around with Git and Mercurial. I can say with confidence that I am impressed with the promise of DVCS.
Eric Sink on the other hand (who I came across originally while looking into Micro ISVs) is certain we do need another DVCS – as he has created Veracity and has kindly offered to give out printed copies for free. I just requested mine.
He recently posted (in three parts) a video covering Veracity.