From what I understand, the differences between the 100/2.8 macro
lenses and the MP-E go quite a bit beyond magnification range. The
first and most obvious difference is that the MP-E is manual focus
only. However, I think you still get focus confirmation. The second difference is that the lens does not focus to
infinity. In other words, it is a dedicated close-up lens. A third,
more subtle difference, is that the MP-E does extend when
focusing/selecting magnification, which means one must be more careful
not to disturb your subject. Fourth, I am told that the extremely short working distance and the much smaller effective aperture means using a macro flash setup is pretty much mandatory at high magnification, especially so if handheld.


Now I'm not trying to discourage you
from *testing* the MP-E, in fact, I'm all for it. But just from what I
can tell from this lens, it is not a replacement for any EF macro
lens. It is just *so* different. Should you rent it, what I think you will soon discover is that it is almost nothing like shooting with the 100/2.8 macro. Try it out--I myself plan to someday, but as it is, I'm pretty happy with 1:1. I've measured an effective print magnification ratio of approximately 35:1. If one could use the 7D + MP-E 65mm, I think it would be possible to obtain effective print magnification ratios as high as 280:1.