Final stand is an awesome Buff,
and as @Lunix Vandal said, it keeps you alive no matter what until it expires. You can use some ways to gain Final Stand like having a Valentine as a support with trauma center Marquee Ability (This changed my playing style and my life, belive me), so in a situation you are close to lose, you get a second chance to make a comeback.
But be aware, some defensive characters or teams uses this buff as well, or even the PF event can give it aside variants used, so it can take important seconds of the fight vs an inmortal Meow N forever Ms Fortune, or a team that has a MA squigly in the corner, so some removing buff moves or characters can help you, or even plaining where to kill your enemies, or the order (Like killing the vals firts to disrupt the team dinamic).
Use it to your advantage, because if you get back HP, you can use Valentines support several times, but a 100% HP punch can kill you without activating it, so be careful!