> I'm a little confused as to how this improves on GNU units, which seems to support far more [...]

> is it supposed to be a competitor? learn how to use a new language? I don't get it.

No reason to be confused. GNU units is a great tool. qalculate and speedcrunch are great, too. Insect is just my take at it. Each of these tools has advantages and disadvantages. As for insect, one of the advantages for me is that it is platform-agnostic and can be used from anywhere without installation. The "You have: X; You want: Y" interface is basically replaced by "X -> Y".