I own a tamron 70-300 VC which is a nice price point and nice quality and has a ring type USM, However the vibration compensation(IS) is jerky when panning by hand. I dont have a nice fluid head but I would assume it does the same thing, which dissapoints me a lot. I wish the IS had a panning mode. Just a bit of info on this lens. I have a feeling a lot of the cheaper lenses have this issue.