The DO lens has the "full version" of 3rd-generation IS; the others do not. This adds bulk and some weight, it seems. But the glass in the DO is just plain DENSE.. What DO saves, is length. But it feels very dense and solid (not bad, but a rather good feel).

Seems to work well enough too. But I find the pictures go soft when backlit, such as shooting birds in the sky, or dark objects against a bright background (exposed for the dark objects, that is).

Love it. Haven't touched my 70-200L-IS since I got the 70-300-DO.

Cheers