Hi,
A Sigma lens will significantly reduce the speed of your camera's auto focus capabilities, if you can live with that then go ahead.
Sigma have some great lenses but don't expect fast performance. I use a 1Dmki and I occasionally use Sigma prime lenses when AF speed isn't an issue.
It would be much better to use Canon's L series with a 1D if you can afford it, they were after all 'made for each other'.