I tried this tip for assembling a “$15 Mont Blanc pen.” At Staples, I bought a two-pack of black Mont Blanc rollerball refill cartridges and a pack of Pilot G2 gel ink pens, then swapped a Pilot pen cartridge for a Mont Blanc cartridge. Per the Instructable, I first had to snip a bit of plastic off the end of the Mont Blanc cartridge to make it fit in the Pilot casing.
It works, although the push-button mechanism of the Pilot sticks frequently; I have to slightly loosen the segments of the pen to “unclick” it. (This could be because I cut too little or too much off the Mont Blanc cartridge.)
The Frankenpen writes smoothly, and the ink smudges very little, which is a fine attribute in a roller-ball pen, but I don’t notice an appreciable difference over the Pilot Precise Rolling Ball pens I currently favor. And really, as I think someone else pointed out in the comments for the Instructable, the point of a Mont Blanc (other than to convey to people that you make a lot of money and are not opposed to sinking some of it into overpriced writing utensils) is that the casing is tooled and weighted for the Ultimate Writing Experience, or whatever the marketing phrase might be.