I used to not like the word compersion because it didn't sound like a real word; but I do use it often now because when talking with others, especially at discussion groups and such, it is useful for describing the situation it defines without going on and on and on. (And I'm a blabber, so I have to watch that anyway)
I could say, "When my boyfriend and his wife share a loving glance and a hug, it makes me feel nice inside to know that they love each other, and that their relationship is strong, and I also have a knowing feeling and a connection because I know what it feels like to get a loving glance from either of them, and it's energy we share,"
Or I could say, "Oh yes, I feel complete compersion for my boyfriend and his wife."
The first option has those at my discussion group falling asleep, checking their text messages or picking their nose because they've lost track, and the second one has them nodding in understanding. (Hypothetically).
So yes, I do use the word in a formal sense, but I don't use it when conversing with my partners. I have NEVER used the word frubbles or frubbly. It reminds me of muppets and bunny cartoons, and my relationships are not made of those things.