I am a bit biased about FilterParams. Kari (@kku) kindly built it in direct response to some of my grumbles about Custom Filter editing. So it adresses many of my personal bugaboos.
His innovative use of color to visualize the dependencies in multi-stage custom filters was a complete revelation to me. It is unique in the Gramps interface. (Row color coding with Tags, Pin coloring in maps and color coding View configuration in Fans offer some visualization of Genealogical data.) It inspired me to develop some color-coded controls to enhance to Addon Manager filter lozenges.
I dearly hope that FilterParams replaces the extremely limited built-in Custom Filter editor. Just the ability to navigate between Object Categories without exiting the interface is enough reason for that.
The feature to Import/Export custom filters was added in response to public discussion here on the forum. (It was nearly impossible to share a complex Custom Filter. You had to describe all the rules, their parameters, and stages. Then the other person had to reconstruct it from that. It was error-prone on both sides.) With FilterParams, you can copy a selected Custom Filter to your clipboard in Gramps, then paste it into a codeblock formatted part of a Discourse posting. Interested readers can copy the codeblock and paste it into their FilterParams Import interface… et voilà!
I think its value could be expanded with some Save/Load (to external files or Notes) features similar to those for SuperTool’s scripts. But some sort of library/grimoire/formulary system for sharing arcane knowledge would be nice too. (Such a system would useful for sharing Place object, Source object, Repository object, and Gram.py script snippets too.)