I have both.

I bought a cheap ($5.) mandolin to see if I would use/like it. And I
find I use/like it A LOT. The 2 pieces go right in the dishwasher. A
lot easier to clean than a fp. If one is slicing a couple of things,
it is more practical than hauling out the big fp.

A chef told me they chop things finely so as to expose more surfaces
for flavor to be extracted. (Make sense?)

Take the whole bunch of celery, lop off the top even, and run it on
the mandolin. Also works for onions, carrots, & bell pepper. All the
slices are the same thickness. Wow.

But no way does it replace the food processor.