*And I just made a discovery. I have a Waterman pen in my office that someone gave me but I haven't really been using because the ink cartridge was kinda sucky. It turns out that the 207 rollerball cartridge can be taken out and used in the Waterman. (Though, honestly, the 207 is perfectly comfortable to write with by itself.)
My wife has a Waterman that we refill with Pilot G2 refills. Just have to shave a bit off the plastic cap.
I just tried this with a gift Waterman that I never used for the same reason. It's wonderful!