If you find the output a bit hard to parse at times (as I do), you might want to try qalc instead, I use it all the time from the terminal to do conversions:
$ qalc
> 3 millilightseconds to miles
3 milliLightSeconds ≈ 558 mi + 1491 yd + 0.1692913386 ft
I'm not sure if it has all the same units as `units` does, but it replaced my use of it entirely as it can do other useful operations as well
