Just saw it pop up on Steam, like they had to offer me a drug I used to be horribly addicted to, had no exposure to in a while, and had gotten fully clean from.

Is it worth diving back in?

I feel like calling it an expansion oversells it... Seems more like a very nice mod package in scope. If you're already modding, its value would depend largely on how interested in the mission features you are. Obviously, you could always make up your own missions. This just seems to provide in-game tools and sharing along with the pre-made historical missions.

Not that there's ever a bad time to play Kerbal, of course.