It depends. If your level 20 campaigns resolve around fighting titanic monsters, eldritch abominations, or other Stuff That Be Freaky, it isn't too useful. If it is more about great wars engulfing the world, armies of undead arising to wipe out the living, or endless streams of evil outsiders, it becomes a lot better.

Your average half-orc warrior with 15 strength and a longsword won't be able to harm you, even with a maximum damage crit. You could take on a thousand without taking any damage.