I actually pushed to use the cards in our home games because the games devolved into rocket tag. We have decent to good optimization in the party so when using modules/APs it was pretty bad, crit weapons with multiple attacks would decimate enemies. The deck was a move to reduce damage output and make the game a bit less lopsided. Our normal GM wasn't 100% sure about it in the start (everyone likes big numbers) but has since implemented it in every game he runs since so even he has come to "like" it and understand what it did for the game.
As for the DC, there are a few cards that are brutal, and that is intentional. The vast majority of the cards are effects or hindrances that don't end the combat. But they did add one card for each attack type that is just nasty, and that is the one that you are hoping to get, that makes investment in to crit feats worthwhile (pulling 3 cards and getting to choose decap? Sign me up, those feats are paying off).
Think about this way, crit damage isn't typically avoidable, decapitation is. If you were playing without the crit deck there would be no save, just "poof" fine red mist. A harsh DC is better than "oh that roll was high, you are unconscious/dead now".
I'll take the save over being mushed with no recourse the vast majority of time (crit cards typically don't kill unless the PC is being stupid), where as that spike from crit damage will catch you unawares and do you in way more often than not.
We use 1/2 hit dice + stat that caused the roll +10 equal the DC .