I use it for both client- and server-side. Do not skip learning JavaScript before learning CoffeeScript, if only to be able to debug CoffeeScript. The JavaScript equivalent of a 1-line CoffeeScript function can be over 20 lines, not because CoffeeScript compiles into bloated JavaScript, but because it is very 'expressive'.

A trivial example (ignore style), project euler problem 1:

  (x for x in [0..999] when x % 3 is 0 or x % 5 is 0).reduce (xs, x) -> xs + x