For that price, I'd just as soon grab the Sigma 70-200 2.8 OS for $150 LESS. I've actually used the Sigma before, and the particular copy was ridiculously sharp.
Considering the collar is ANOTHER $200+, you really aren't getting all that much for your money for an f/4 lens.
This should be around $1000 - collar included.
You can't really tell from the picture, but that is a gigantic lens.
Looks nice...I just can't get past Canon's ergonomics.