To answer your question stillwaiting, the JSD takes about the same amount of time as the PhD in most cases. The coursework may take only one - two years, but the candidate may take many more years to complete the dissertaion. I would guess that most finish in 4 to 6 years total. Very few schools offer the degree and those that do are highly selective, normally only accepting those who hold an LLM form the same intstituion to which they are applying. It's true that very few people actually pursue the degree as it isn't really necessary for any particualr employment, academic or otherwise.
The other more common post-JD is the LLM, which you may already know about. This degree can be obtained if you wich to do study and research into a particual area of the law, and can improve a lawyers viability as a potential law educator. It can also help to oercome the stigma of a JD from a "low ranked" school. You will notice that many law professors may have JD's from sub top 20 schools, but have an LLM from a top 20. It can also be helpful if you want to make yourself more desirable to employers in a particular field of law.