I have the MP-E-65, an MT-24EX, and I can confirm that at 5x the lighting does come in at a pretty sharp angle. But you can still get front lighting on your subject.
Working distance from front of lens is about 1 to 4 inches (going from memory) at 5x to 1x respectively.
Depth of field is miniscule, about a millimeter and a half at 5x and open f/stops. The shallow DOF, short range, and large size of the thing when it is at 5x (it zooms like mad, I believe the length triples at 5x) make it a pain in the butt to use. I like the lens, but I've got bugs on the brain! With anything other than a static subject and a tripod shot you will lose MOST of your shots because of DOF problems or the subject moving out of the frame. But some of the ones you keep can be amazing. I have head-on shots of harvester ants ("red ants" in much of the central and southwest USA) almost filling the frame.