I've never flown in one but I have spoken to a few guys that have, and apparently the dual rotor configuration causes lots of vibes. Even more than the traditional configuration.
The Chinook (so I'm told) had something like a spring mounted floor to minimise the vibes transmitted to the passengers. Lots of these guys said that you felt very sick after flying in one of these blenders.
I understand there have also been one or two disasters where the synchronisation was lost between the meshing blades and both rotors smashed themselves to pieces.
Not good for public relations.