I'd have to say I don't reckon it's the worst I've got... The manual focus rings on my 35-105 and 70-210 push-pulls take that prize - the 70-210 is particularly awful and very difficult to use with any degree of precision...

By comparison the 50mm is quite smooth - not as good as the 50mm Nikkor I also have - but definitely acceptable though the ring's a lot narrower than I'd like...

Worst I've ever had has to be the Nikkor AF 28-105G kit lens that came with my F65. Aside from a build quality that made even the worst Canon lens look quite solid (and I'm no fan of Canon construction standards) the focus ring was little thicker than the edge of my Cokin polariser and gritty as hell. All in all the most appalling lens I've ever owned...