I have both the Canon 100mm (non-L) macro lens and the MPE-65, with a 5DII and MT-24.
I find that the 100mm lens is essential for larger bugs, and it's also useful for other things such as highly detailed portraits and landscapes. Even with a FF camera, it's easy to need less than 1:1 magnification (large butterflies, larger hawk moths, etc, etc). The increased working distance provided by the 100mm lens is also helpful in many situations. Often, I will add a couple of extension tubes to the 100mm lens and thus I can use it somewhat beyond 1X magnification. However, with tubes it will still focus to roughly 0.25X mag, so it is still very useful for large bugs.
But I very much love my MPE-65 for its very quick adjustment of magnification.
I've been considering adding the 100mm L lens to my collection because of its IS feature, which would be useful for non-macro applications, but the cost is a consideration - I would probably get more out of other lenses for non-macro purposes. Also, the detail provided by these two lenses is similar as best I can discern from others' posts - no substantial macro advantage that I am aware of.
I wish the MPE-65 had a magnification range with my FF camera from about 0.5x to about 4x. Then it would never leave my camera during macro season.