I was also wondering, as this might be one of my next lenses, is the AFS version's collar really that bad? From what i've heard its not really that good so I was thinking about the idea of an aftermarket replacement. What about an earlier AF version- how is its collar in comparison? I might get the earlier one for cost reasons.
I find the collar to be adequate on mine and not such a big deal on faster shutter speeds. Thus on a monopod the lens has not failed me yet. However I think for having full use of the lens from slow to fast (and tripod use) it may be a good idea to replace the collar over time. I think I will change the collar (to the Kirk) but in the meantime a good hunk of solid rubber suits me fine.
I use the 300/4 AFS with Kirk collar on a Manfrotto 680B monopod with a Manfrotto 3232 head as modified here: http://www.reallyrightstuff.com/projects/index.html A very solid rig. I like it a lot.
