It was the LD (model 572D) but has now been changed to a Di (has
'digital' suited lens coatings), I've got the 572, and was looking at a
friends new Di last night, cosmetically nicer, I didn't look side by
side for lens quality, but nothing jumped out at me. Still as slow at
focusing, still plasticky.

I don't know the Nikon lenses, but reading about Canon lenses in the
same range, didn't offer anything over the Tamron except a larger hit on
the wallet, the only real quality jump came if you bought an L series
lens.

Nikon D lens transmit the distance information to the camera (and the
flash) . On "G" lense the aperture is set in the camera (no aperture
ring).  A lens can be both D and G (18-70).  My 1.5 50 has an aperture
ring - it is not a G.

Jack

The D and G designations apply to Nikon branded lenses.  Optically,
the D version is significantly better than the G version.

The Tamron LD lens gave results comparable to the Nikon D (better)
version but at a much lower price.  The Tamron LD has now been
upgraded with improved coatings on the rear element to reduce ghosting
and flare when used on digital SLRs.  The upgraded lens is the Di, and
it offers excellent value.

All these 70-300mm lenses (Nikon and Tamron) are made in the same
factory, by Tamron.  They offer very good performance in the 70-200mm
range but perform less well as focal length increases beyond 200mm.
Think of them as very good 70-200mm lenses with the ability to shoot
at 300mm in an emergency, or when ultimate optical performance is not
required.