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