There is a huge list of things that I am much more afraid of than a simple Death Attack. If the DM is going to randomly attack you with save-or-die effects, Death Attack is not the sharpest option in RBDM toolkit. There are plenty of insta-kill spells and effects that have a scarier range, higher DC, and don't require someone to stare at me for 3 rounds.
I've played an Assassin before, and I can verify that the DC of Death Attack is not in any way awesome, but it's also not craptacular. I used Str as my primary ability, with Int as a secondary (Dex was low; AC doesn't matter at all if they can't see you, and I was melee focused). I also took the Ability Focus feat to give an additional +2 to the DC. Like any Fort Save effect, it was very effective against anything that had a low save, and crappy against anything with a high save.
I've played an Assassin before, and I can verify that the DC of Death Attack is not in any way awesome, but it's also not craptacular. I used Str as my primary ability, with Int as a secondary (Dex was low; AC doesn't matter at all if they can't see you, and I was melee focused). I also took the Ability Focus feat to give an additional +2 to the DC. Like any Fort Save effect, it was very effective against anything that had a low save, and crappy against anything with a high save.