I own the Canon 70-200 f2.8 IS, regarded as one of the finest, sharpest zooms made. I also have a 1.4 tc II which brings it to 280mm. In addition to that, I have the 70-300 DO IS, which I find to be an excellent lens. But I digress.
I just got the Canon 100-400 for Christmas. I too was afraid of some things I've read regarding softness at 400. I now find that to be totally unfounded, at least with my sample of it. I think it's worth every bit of what it costs, the IS works wonders. I've handheld it at 1/25 with excellent results. And many, many people here have posted stunning results with it.
I'm in no way denigrating the Bigma. The only thing I'll say is, quality wise, one usually get what one pays for. I've handled the Bigmas and they feel cheaper in comparison. Not cheap, mind you, but just not the same quality.
mark