I had an unfortunate mishap with my Pilot Metropolitan at work, and destroyed the nib. I dropped it (cap off), onto a tile floor, it bounced off the nib and "double-tapped", bending it in two different directions. Needless to say, the nib was mangled. Wanting to try a different type of nib, and not willing to buy another metropolitan, just to cannibalize, I decided to buy a plumix and swap the nibs.
I did not try out the Plumix first. I immediately dismantled it, cleaned both pens with plain water, and re-assembled the plumix nib and feed together on the Metro, using the same half-spent black cartridge I was using previously.
After the first few strokes I noticed it would randomly skip on the first 1/10 inch or so of a downstroke, and randomly on side strokes, then start flowing. Now, I have never used an italic/stub nib before, so maybe I'm doing something wrong. I was trying to keep the nib flat and straight on the paper, and not roll it to one side. I was also writing very slow. Even so, it was still acting up.
I've read reviews where many people have said the plumix nib is one of the smoothest Pilot nibs out there. While actually flowing I would say its very smooth, but the skipping issues ruin the experience.
I do not have a loupe, but I do have an SLR camera, and multiple lenses, so I tried the "reverse-lens" trick. It was hard to see, as I probably shouldn't have used a 28-200mm zoom lens (very small opening at infinity focus), but I could not see any misalignment of the tines.
I did end up buying another metro to use as-is, and both Mediums are/were buttery smooth. It's a shame I'm having trouble with their stub.