If you fancy a bash at extreme macro, then this is the lens for you. Most "proper" macro lenses offer 1:1 reproduction, which means that the image of an object on the sensor is the same size as in real life. You can fill the frame with a postage stamp. But the unique Canon MP-E 65mm starts at 1:1 and goes all the way up to an amazing 5:1. At that magnification, you can fill the frame with an object that's only 4mm across.
Using this lens is deceptively simple. There's no autofocus. You just adjust the lens to the magnification you want, move the camera closer to or further away from the subject until it's in focus, and take the picture. Or you can set the working distance and then use the magnification control as a focusing adjustment and turn it to bring the object into focus.
But using this lens is also deceptively difficult, for several reasons.
If you're tempted, Wayne Schmidt has written a very useful guide. Are you up for the challenge?