The idea of immortality is incredibly unappealing to me. And it really does completely defy the balance of life. Earth would become ridiculously overpopulated to the point of catastrophe, we'd run out of basic and vital resources, destroy our planet even further than we've already done, and no doubt there would be unintended side effects. Plus, class disputes would likely get worse since the rich would get the substance first and in the most abundance, leaving others without.
Immortality would be a fiasco and basically destroy humankind.
I'm going to live the life that I have so that when my time comes I know I did what I wanted and that was that. Immortality would end up being agony, because honestly, I see no point in living for centuries on centuries. And that's exactly why the idea is stupid... it's pointless. There are no positive reasons to be immortal.