A specimen of this lens is passing through my hands, and I had the opportunity to give it a quick whirl last night. I compared it with a Nikon 20x/0.40 ELWD finite objective--the same model Rik tested this JML against. Not that I doubted Rik's findings--just had a hankering to see what this inexpensive JML lens would do in my own hands. My comparison was less rigorous than Rik's, but produced similar results.
NB, I compared both lenses on bellows with the same amount of extension: 200mm from lens shoulder to sensor. Since Rik found that the JML produced 20x magnification at 222mm extension, I was using it at slightly lower magnification, and perhaps a bit out of its sweet spot.
From this exercise, my takeaway: In the center, resolution is about equal between these two lenses. The JML's sharpness degraded quickly away from the center, while the Nikon held up well to the edges. This might not be a huge detriment to photographers doing portraits of insects or other centered subjects.
Chromatic aberration is very different between the two lenses. With the Nikon, CA is axial (purple fringing), and can be troublesome to correct in post. With the JML, it is transverse, and easy to correct in post (as Rik said).
Neither of these lenses really competes with the Mitutoyo MPlan Apo 20x, which is remarkably free of CA and has about twice the working distance. But the Mitty is an infinite objective, and so requires an additional converging lens. It is not suitable on a straight bellows setup.
Putting it into perspective, this JML costs $35. The Nikon 20x ELWD is, if you can find it, about ten times the cost (I see one listed as BIN for $340, which seems a bargain to me, as I paid $500 for the copy tested here). The Mitutoyo 20x lists for $1900, though it can sometimes be purchased second-hand for a third to half that price (I see that one sold recently for $750).
At this price, I think this JML 20x is an outstanding bargain. Respectable 20x for $35? This is the price of a cheap diopter. Anyone working on a bellows who doesn't already have a way of getting to 20x might be well-advised to take a look at it.