"Beyond Monks" and Netbook of Feats
Having played a monk from level 1 to 13, I voted "Limited Usefulness", and I give Stunning Attack only that high a rating due to some of the feats in Chainmail Bikini's Beyond Monks and the Netbook of Feats.
Beyond Monks has a number of feats that require using a stunning fist attempt to activate. For example, Eagle Strike gives you a +4 bonus to damage on a charging unarmed strike when you give up a stunning attack, and Impossible Balance gives you a +20 bonus to balance checks for a round, for each stunning attack you give up. If your monk has Wis 17+, you can take the Flashback feat, which lets you spend a stunning attack to gain a memory flashback from you childhood, training, or even your ancestors; this grants you the use of a martial arts feat (from a bonus feat list given elsewhere in Beyond Monks). There are several other valuable feats, all of which make Stunning Fist less useless.
The Netbook of Feats has a few interesting monk feats. One that my character took at 12th level was Penetrating Ki Strike, which lets you spend three uses of Stunning Fist and make your next unarmed melee attack as though it were a touch attack, bypassing your opponent's armor, shield, and natural armor.
So, by adopting some feats from non-WotC supplements, our campaign made the Stunning Fist ability not as completely useless as it had been.
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