I have one. It's nice and compact. Lighting is tricky in such a confined space and it seems vital to keep any light from hitting the front element or contrast starts to vanish. If you stop down at all, triangle bokeh is very bizarre. Not an issue if you are stacking and have no out of focus highlights though. Diffraction starts to kick in pretty quick, so you don't really want to stop down much anyway.
I don't find it nearly as sharp as my dedicated macros (DFA 100 & 50), or my 2x magnfication combo (dfa100+DCR250) or even my 3x mag combo (dfa100 with a reversed A28/2.8), but I'd put it above my previous 4-5x combo (the A28/2.8 reversed onto a bellows).
It's not easy to use, an automated rail would probably be a good idea. I have a manual velbon clone, and it's borderline at 4x magnification.
The above equipment represents the bulk of my macro'ness. I'm really a rookie at greater than 2x magnification. If there's something specific about the Mitakon you'd like to know, please ask. I've had it for about a year, but part of the reason I bought it was for a project I was still gathering materials for and will be shooting in the next few weeks so I should have things ready to go.
This was 8 stacked shots under studio lights (caterpillar was naturally frozen on a snow bank and brought indoors for a quick shoot). This is the full frame of the k5iis at 4x mag:
Edit- to add a 1024x701 pixel section of one of the slices that went into the above stack: