now as usual OP should remember - you can always get a bad copy of the lens - so by all means either consider buying in store to select from many copies _or_ calculate your troubles if you will need to send it back to the merchant or to Sigma service center to exchange/fix/refund ...
the author of the link above mentioned that his copy had some defective focusing unit (which is a group of optical elements housed inside the barrel in a special assembly which keeps/moves them using some gears, drived either by screwdrive from the body or if you will engage the MF clutch by rotating the focusing ring with you hand)...
now it looks like (at least how I read his words) that it was not an isolated incident and Sigma is replacing those focusing units inside the lens... now what can be replaced ? he says it moves (focuses) faster now - so either it is "lighter" (think some glass/metal was replaced by plastic or the actual size of the elements in focusing group was reduced - but all that means a major change in optical design, not that easy - right ?) and/or the gears design (or how it was assembled - gears I mean, not optics) was defective and while focusing group itself (optical elements and assembly that holds them) remained the same - just gearing (mechanics) was replaced... pure speculations though
