Well,
It sounds like I may be the exception in the group, with my boldness.
My opinion is that the penmills can and should be sharpened. Not just that, but that they can be greatly improved by removing the rear shoulder/planing surface.
So, what did I do to my poor mill you are asking.
1. Removed center spindle (that is one really wierd hex size btw)
2. Using 1" verticle belt sander: as carefully as possible taper down the four non beveled rear surfaces of the cutters. This was done until the rear bevel/shoulder was just removed, leaving a clean cutting edge. (note, care was taken to attempt to leave each of the cutter blades at an equal height, but it _was_ just done by hand)
3. Using a fine dimond file or stone, hone each cutter to whatever level of sharpness you can tolerate.
Results: A mill that really cuts, and really trims down the material quickly with much less downward force on the blank.
My mill had gotten so dull that I was heating up the blanks and not really cutting anything. I had already tried sharpening with minimal material removal with a hand diamond file, but had reached the point where I was either going to get a more acceptable result or trash the mill and go buy another one.
The milled surface seems flat and square enough to me, no different than before really, but next time I sharpen the thing I will use a straight edge across the cutters and measure each cutter with my dial calipers, in hopes of improving on a situation that I am VERY happy and satisfied with right now. This thing really cuts and I have not sharpened for quite a while now, since I did the modification.
I remember there was some discussion a while back on the yahoo group, and a person who rigged up a jig to hold his dremel tool to give the cutters a clean mini-hollow grind on the back side. I thought it was a great soloution, but I did not want to go to that extent right then.
Conclusion:
I suggest getting rid of all or at least some of the rear shoulder on the mill cutting edges. (differing opinions welcome [

] )