Seems the Sigma APO 120-400mm f4.5-5.6 DG OS HSM Lens is comparing very well against the Canon EF 100-400mm f4.5-5.6L IS USM
A well known Japanese company that does lens comparisons has summarised their comparison with "The difference is 'order of magnitude'.Sigma gives the more successful images in terms of the image stabilisation and sharpness."
The full report is available here if you can read Japanese
EDIT: The Sigma can be had for under $1K, where the Canon is usually just over $2K.