I tried out 4 copies of the EF Sigma 100-400mm and reported the results here. This was my last post on it;
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"Photozone aka optical limits has finally reviewed the lens:
http://www.opticallimits.com/canon_eos_ff/1045-sigma100400f563c
They found what I found in the 4 copies I looked at courtesy of the local dealer; the IS in the viewfinder is terrible and not much good in practice. Their summary is: "Thus even though the Sigma is quite capable in optimal conditions, we'd suggest to look elsewhere."
My experience of the Tamron 100-400mm f/6.3 wasn't good either. I made the mistake of buying one because I thought it would be light for travel but eventually sold it as the tracking of birds in flight was hopeless. The Sigma and Tamron 150-600mms are much, much better and really worthy of consideration."