I purchased my copy from Amazon a few weeks ago now. I have never purchased a third party lens in my entire photographic lifetime (30 odd years) and felt very nervous about doing so. I first purchased the Canon equivalent, but the build quality wasn't up to my expectations, although it delivered some nice images. I sold it on and held back for a few months, finally taking the plunge and going for the Sigma lens.
There are numerous reviews about the optics and build of this lens which are mostly positive, and do not need any further input from me, though I was concerned over the back and front focus issues some people have had with their copies. But that issue can be fixed by Sigma very easily via calibration, if you can get over the inconvenience of sending it in to them.
My impressions when I first took delivery were mixed. One minute I loved it, the next I was wondering how long the finish would stay pristine. I reserved judgement and just played with the lens awhile. Anyway, over the course of the day I began to love it. I photographed a focus test chart and am pleased to report that I did not have the dreaded front or back focus issues others have had. The lens is sharp with good contrast, delivers a wonderfully smooth bokeh when opened right up, feels and looks good on my 5d full-frame, and has a build quality the knocks the Canon 1.4 into a cocked hat. Yes it's quite heavy, but not uncomfortably so; and yes it is bigger than the Canon 1.4, but only by 1.5 inches in length with hood attached, so not that long really. I think the build is as good as any L series Canon lens, and I currently use the 24-105 most of the time, though not being a scientist I wouldn't argue the point about the quality of the optics. That said I can see this lens playing a big part in my future photography trips. Well done Sigma.