I'm looking at getting it one sometime in the mid-future. I finally have an E-TTL flash, so I won't have to guess on the exposure anymore
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The MP-E can go from 1:1 to 5:1 (5 times life size). However, it is ONLY a macro lens. You cannot focus any further than 1:1, so you couldn't use it to take a picture of a bird or something like that.
I've never used one, so I can't give you much information about the ergonomics of it. However, I have used a reversed 50mm lens setup, and I can tell you that nothing is harder than using that setup
. The DOF is razor thin, the sharpness at the edges is abysmal, and you have to be about .00000000001 inches from your subject. The MP-E has aperture (read: DOF) control, has reasonable working distances, and is sharp throughout its range.
As LordV and Bald Eagle said, it isn't a lens for beginners. But, if you've done macro for a while, and like it, you need this lens (at least, that's where I am right now).