I love the 20 = double damage randomness - especially when it splatters PCs. But then again, I love dangerous, crazed combat that could lead to death at any turn.
When a 3D6 attack suddenly becomes 34 damage and the demon standing in front of you gets his head lopped off, that is an awesome moment. Last year, we ended a tournament with a massive crit on a dragon that got the table howling with excitement. At the Gateway convention, we lost two heroes to evil elves with great crits with both swords which left the table gasping when the 5 PCs suddenly become 3 PCs in one round. My group still laughs about the TPK thanks to goblins in the darkness getting 4 crits in a single round...and then 3 more crits on the next party who were also a TPK.
But sadly, awesome moments must be sacrificed on the Altar of Balance to make sure little Timmy doesn't keep crying when his character dies.
Too bad we just can't crit Timmy.