"Dry" Dewars ?

[Jens Heese]

Hello, 

We are considering the purchase of one of those "dry" cryo-dewars.
After a rather bad experience transporting a conventional dewar
to and from (not) a not to be named synchrotron, I'd like to
ask for opinions and experiences with dry dewars. Did they
work ? How long do they keep cool ? And above all experiences
trying to get things shipped to and from sites.
Thanks for your help.

M.