Macro with the MP-E 65mm

I currently use a Nikon D5100 and Nikkor 60mm with tubes and/or close up attachment for macro work at greater than 1:1. I'm considering the MP-E 65mm (650D Body) as Nikon have nothing equivalent and wondered if there was anybody with this lens who could give some idea of how it performs in use.

I will be looking to use it mainly in the field to photograph micro fungi but could also be using it indoors/studio with the same subject.

I would think that the DoF would be very narrow but how does the lens cope at smaller apertures e.g. f16. Also how is it to use for focus stacking without a focus rail.

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JohnB
http://farm4.static.flickr.com/3182/2298450593_a65e0ee642_m.jpg
One photo out of focus is a mistake, ten photos out of focus are an experimentation, one hundred photos out of focus are a style. Anon