I have not used one yet but take a look on ebay I found them there for about cost. As far as I know Mack sells them to places like B&H for aboud 1/4 or 1/3 of the list price. however I did buy my Mack EW for list and from the place I got my d70.
I bought a Mack warranty for my 70-200 f/4 L. I can't tell you how it is, because I haven't had to use it, and hopefully I won't. Registration was easy though.