First, the lens is larger and heavier than either my EF 17-40mm f/4L lens or my EF 24-70mm f/4L IS lens. Second, this is the first rental for this particular copy and the purchase price is only $21 less than the new retail cost of $999 which would include a warranty from Canon.
Third, the lens has some circular flare with the sun in the frame which I've seen on online review testing and which I ran into this morning. Here is some rainbow flare at 16mm and f/8, which is typical, apparently.
In any case, I have until Friday to evaluate whether this lens is worth the extra money over the 17-40mm lens I already own. Here are some other shots from this morning's walk.