Not sure if this is on purpose or not but:
These icon sets don't give the icons names (well, they have names but they're new/arbitrary names like "0-20Black.png") which means the icons won't override existing toolbar icons when installed the set is installed.
If you check out Christiaan's Vista Icon Set you'll see that if you install it and then move it to top of the list then it automatically changes all your toolbars to use the new icons (unless you've edited/created buttons so that they explicitly point to particular icons or sets instead of implicitly using the top set).
On the other hand, with this set you've got to edit every button individually and if you then decide to change back you have to edit them again.