They are one of those things that make life a lot easier. Problem is I've only seen them made by Bison in Poland and they are really spendy most of the time.
When I bought mine, Enco was having a special and I got it for less than $200. Not the case now. Here's a link to the Enco page listing them:
http://www.use-enco.com/CGI/INPDFF?PMPAGE=417&PMITEM=271-5304
There is currently one listed on eBay that's even a little fancier than mine in that it is a set tru mounting:
http://www.ebay.com/itm/3-Jaw-self-centering-rotating-4-chuck-set-tru-with-Morse-taper-shank-Bison-/261013254970?pt=BI_Tool_Work_Holding&hash=item3cc59a333a
A couple guys bidding on it currently. I think it is a steal at the current price but I bet it sells a lot higher.