Well I ordered mine today along with a RC2 adapter.
I have been using NN5 + Manfrotto x55 pro + ball head for years taking nadir shots with the center column at 90 deg.
It's a bit cumbersome, but works perfectly if you level the nadir shot (2 way hot shoe bubble level is indispensable).
With both Hugin and PTgui having viewpoint correction, it just works, even though my the height of the nadir shot is approximate.
Mind you VP correction only really works on flat surfaces (setting manual CP is recommended, 3-4 should be enough), but then again smart masking works wonders.
The nadir adapter used with an 2 x PD tripod offset should yield perfect nadirs as the camera should be close to the original nodal point.
The reason I decided to finally get the nadir adapter is speed, but also as a help to mask out the pesky long shadows that haunt early/late panos in the sun.
Currently have a bit of a nightmare of a pano to clean, as the shadow is a few meters long and is all over various boulders, no flat surfaces :|
Enough is enough, getting a RC2 adapter too so I don't have to carry a bunch of cameras with me.
My only niggle is the possibility of flex with the adapter, but I hope it's not much.
Also, kudos to Heinz for the nice informative shots for the nadir adapter workings.