Ink cartridge review, to be more specific.
I've always had an affinity for fountain pen ink cartridges, but have never truly embraced all of the options available. Until this point, my most used cartridge has been Pilot Blue Black in most any Pilot pen I own. That's right; they are proprietary. Same with Sailor, Platinum, and Lamy. Proprietary is fine, as long as there are options.
If you want even more options, international-sized cartridges give you that. But they are often limited to short international cartridges, made primarily for pocket pens with shorter barrels that can't accommodate a full-length cartridge. Pelikan makes an excellent standard international cartridge for their Edelstein lineup, but that's about it.
For mixing and matching colors with your compatible pens, short international cartridges are where it's at. And thankfully, that market has blown up over the past several years.