Since you are talking about local "coastal cruising" I see no big negative if the price
is attractive to you, and you handle the sail well (not abusing it with a small crew dragging it over sharp things on deck
Offshore Cruising or Circumnavigation?
But, if someone reading this thread (because of the title) considers a mylar sail for offshore
long distance cruising (such as a circumnavigation), I think the issue is "repair" when needed in remote
locations or underway and your crew's ability to make the repairs
last and hold for another 2,000 miles or more, while depending on that sail to get you where you want to go.
Circumnavigators sometimes take several years to finish, and because of that I think it would be preferable to have a nice, quality, good condition, Dacron "Cruising" sail that could be sewn up easily by a two person or singlehanded crew, if it blows during a crossing and the sail requires some repair by the crew.
That's how I see it.
Hope that helps.