> Principia is released on every new moon with whatever features and bug fixes are ready at the time. This ensures relatively timely improvements and bug fixes.

This may be the funniest/best way to say "we have a roughly monthly release cadence" that I've ever seen.

Any idea how well it works? Are there trade offs, or is it all upside, physics-fidelity-wise?

It worked really well when I used it. It replaces a bunch of the stock navigation tools, and takes a bit of learning, but really seemed to be a good n-body simulation engine. It's been well over a year since I touched KSP though, so I can't speak for the current versions of either Principia or the base game.