There aren't big difference between old and new versions:
- MK2 9 blade diaphragm, MK1 have 8
- MK2 18 elements in 13 gropus, MK1 16 in 13.
- MK2 82mm filter, MK1 77mm
- MK2 Inner focusing with USM, MK1 front focusing method
- MK2 805g, MK1 950g
So is $1000 worth to cover those changes
After having five of the old version, I gave up on them. They were ok, but not so good that I'd keep one. They also look better on full frame, which tends to be more forgiving of lens flaws.
Lens reviewers have had much the same experience, some having had to test multiple lenses in order to find one that was even close to the Canon mtf curves. Even then, the design itself results in excessive curvature of field, which means that, at wide apertures, a flat wall will only be in focus at the edges or the center, but not both. They are better at f/8, but did you pay all that money to get a lens that you had to use at f/8 to hide the curvature of field in depth of field?
The new one better solve those issues, considering the price.