Is this in a state where it's worth buying? What are the chances of it leaving In Development in a complete state or of it remaining partially unfinished?

Worth every penny I spent on it, even if it's never more than it is now. :)

Devs seem to be releasing regular updates, however, and we now have maybe 2/5ths of the promised world map to explore (and that portion is a lot).

The development is very active with regular updates. It's due to release soon I think. It's a bit janky, but honestly it's an amazing game. Ridiculously good.