Below Average:
This feat is actually a reasonable feat for a two weapon fighter who is facing an opponent who uses a shield. It lets the character translate their primary off hand attack (striking a shield is an opposed attack roll so it won't succeed with a secondary attack off hand attack granted by Improved TWF) into a primary hand attack at against an armor class that is probably significantly lower (especially at high levels).
However, in most campaigns the majority of a character's foes will not be using shields. This immediately limits the usefulness of the feat as it will primarily be useful against some some fighters, most clerics, a few barbarians, and some monstrous humanoids. Against wizards, most barbarians (who generally use two handed weapons), monks, sorcerors, and druids (who will probably be wildshaped if they're involved in melee) as well as the large number of outsiders, dragons, beasts, animals, magical beasts, undead, and abberations, the feat will not be useful. In fact, the range of foes that it is useful against is far lower than improved trip, improved disarm, improved sunder, or other feats whose utility is limited by the number of foes they're useful against.
It is also limited in its utility. Even if you're facing a foe against whom this feat can be used, it still doesn't give you too much. Since attack bonusses usually scale more quickly than armor class, it will usually be harder to pin your opponent's shield than to hit your opponent.* Assuming you succceed, you then have to make an AoO (and hit) in order to gain any benefit. Even then, the primary benefit you gain is the ability to use your whole strength bonus and the enchantments on your primary weapon rather than half your strength bonus and your secondary weapon's enchantments. Even if you have a 24 strength and your primary weapon has +2 more enhancement than your secondary weapon, this will only amount to 5 extra points of damage. (And since the odds of not damaging your foe at all are probably higher when using pin shield than when not, this will be even less). In addition, having used your AoO on your opponent for this round, your opponent may now attempt risky manuevers that would ordinarily provoke AoOs (disarms, grapples, sunders, etc) without fear of reprecussions.
That's not to say that the feat is useless. A fighter or ranger who partners with a high level rogue or Order of the Bow Initiate who has the opportunist ability would gain tremendous milage out of this feat. (There aren't many feats which enable you to generate your own AoOs against opponents; most of the time your opponent can avoid them which he will if he figures out that your friends are opportunists. . . .) Similarly, it has high utility against defensively buffed combat clerics. Except in combination with the Opportunist ability, however, this feat does not compare to other ones like Expert Tactician, Close Quarters Fighting, Improved Critical, Shield Expert, Weapon Specialization, Improved Two Weapon Fighting, Power Critical, Ride By Attack, Whirlwind Attack, Expertise, and Spirited Charge. Even a high level human fighter will usually have better things to do with his feats.
*This assumes that you're up against a fighter. If you're fighting a buffed cleric (who's not using Divine Power), their AC may actually compare favorably to their attack bonus. A 9th level cleric with full plate and a large shield can easily have an armor class of 30 (Magic Vestment on each and Shield of Faith for 4 points of deflection bonus). On the other hand, his attack bonus is probably only +9 or +10.
