Chances are you're looking at this lens out of curiosity - you might assume this lens is like a fisheye or exotic lens with no useful purpose. If you assume the preceding then you are wrong. This lens is an entry into another world - the only lens in it's high magnification class to be easy enough to handhold.
It's similar and is likely very inspired by the Olympus macro system. Who had an autotube system and 38 2.8 bellows lens (3-6x magnification). I only have the 20 f/2 and bellows unit (3-15x magnification) and while much better wide open and performance in general it's very hard to use requiring tubes or bellows attachments.
With some practice and technique (such as the left hand technique - hold flower with left hand and balance camera on hand or leg), the mp-e can be tamed and really shine for insect shooting. Photographing bugs takes a lot of patience and learned instinct in how to approach them.
High magnification flowers and rusted metal or soap bubbles or... any object you can think of that has a complex macro structure - are extremely rewarding to find and compose.
It's my most 'fun' lens. Looking through the viewfinder and seeing a macro "landscape" in front of your eyes that you would never ever have imagined before being there - is philosophically rewarding.
Perhaps this is canon's most philosophical lens.
Because after you use it you'll learn to love the small world beneath your feet you never paid attention to before.
You might even watch out for ants instead of trying to destroy them with your superiority.
If photography is about seeing the world in a 'different' way (time itself - shutter speed, the physics of light, the big and the small), then surely this is one of canon's most important lenses to seeing the world in a different way.