A nat 1 is always a failure unless there is no penalty for failure. It is not necesarilly a spectacular failure unless it caused you to miss your DC by a significant amount.
A nat 20 is always a success even when it should be impossible to succeed. You may only barely succeed or do so with flying colors based on how much you overcome your DC.
furthermore, I rule in combat that if you roll a nat 20 to hit, you confirm for critical as normal. However if you nat 20 on the confirm you automatically do max damage. Furthermore you roll once more, if you nat 20 on this last roll you instantly kill or subdue your target depending on the damage type.
Normally I also rule that three nat 1's in a row anywhere also kills your character but this game will be difficult enough without that rule.