Jack,

I have the 521 and it does all I generally need, in fact I totally prefer to do all my focusing through it!! I find the focus ring to be so short in movement it can be a pain. My understanding is the pro it is like storing a rack focus point, push it and it goes to the prior focus point. I think that would be useful now and then, or for someone who shoots shorts mostly.

Thanks Dale – the "rack focus" feature is built into the XL2 as a preset, but it's another thing I would rather not reach for. Was there a website where you got this info, or did you ask a (gasp!) human being? I'd like to know as much as I can before I purchase, but there seems to be precious little info on the 521 models.