I have used the Bigma for well over a year with my Nikon D100 and D70, mainly in ship photography, and sometimes in quite severe weather conditions where, by necessity, the lens is always mounted on a large Gitzo tripod, often with a bag of stones hanging from the base.
Strange to think that this whopper has almost become my standard lens!
I find the Bigma is quick and responsive and, despite all the drawbacks caused by weather and changing light conditions, the image quality is quite remarkable.
My comment on the weight stems from the belief that you don't get this type of performance from a lens built of lightweight materials. More weight - greater stability - better image.
Mine has needed one repair, that is when an amount of play developed in the main barrel, often shutting down the AF function. This was rectified very quickly by Sigma Imaging (UK) Ltd. at a very reasonable cost with a quality of service I found excellent.
My only mumble would probably be the zoom lock. I feel I can't rely on it and the switch being too small and flimsy.
Apart from that - an excellent lens!