What level rogue are you talking about? 4th-6th?
On a 14th level rogue, that's more like:
Primary attack: make a Fort 37 or be staggered! (And 2 points of strength)
Off Hand attack: make a Fort 36 or be staggered! (And 2 points of strength)
Iterative attack: make a Fort 44 or be staggered! (And 2 points of strength)
Imp TWF attack: make a Fort 35 or be staggered! (And 2 points of strength)
DM: OK, he's staggered already so shut up about the fort saves.
As to why the feat is not too good? It is too good. FAR too good. If it functioned once per round and the DC was based on something like half character level plus dex modifier (or int modifier or strength modifier), it would be more in line with the rest of the mechanics of the game. As it is, anything vulnerable to sneak attacks is going to be staggered if they start the round next to an appropriately leveled rogue with the Staggering Strike feat. Either that or they are so absurdly advanced (usually by hit dice using a monster with 4 hd/cr increase or something similar) that they have exactly a 5% chance of failing any save no matter what. (A situation which hoses spellcasters even more).
Of course, all of that is talking about a standard rogue with the feat. There's also the multiclass rogue--a shadowbane inquisitor for instance:
I cast Rhino's Rush as a swift action, Power Attack for 12, smite evil, use my shadowbane inquisitor smite, and charge into the flank.... I hit. That's 109 points of damage and a DC 109 fortitude save or be staggered.
DCs for ordinary combat mechanics should NEVER be based on damage dealt (or a skill check result). That's why.
And as for the mage who was just going to five foot step and cast a spell anyway... at mid to high levels, the mage may well have wanted to suck up the AoO to get out of full attack range before casting the spell.
Ki Ryn said:
But that's not how it will work in practice. In reality it would be:
Primary attack: make a Fort 17 or be staggered!
Off Hand attack: make a Fort 16 or be staggered!
Iterative attack: make a Fort 23 or be staggered!
Imp TWF attack: make a Fort 12 or be staggered!
And that's even more annoying when the bad guy was just going to cast a spell anyway. 4+ extra dice rolls (for this one character's turn) and it doesn't even matter. But since it's an absolute freebie, the rogue is going to ask for every save 'just in case'.
I'm a player so this ruling help me, but I'd still rather have it just be once a round, or reduce sneak attack by 1d6 or do SOMETHING to make the rogue consider whether to use it or not.