Certainly too powerful
It's definately too powerful, and has strange "ramping up" qualities, in that it gets more powerful at higher levels compared to lower levels, due to the interative nature of attacks and magical defense availability.
For example, vs. a fighter5, who might be expected to hit lower AC's like 15 very often, you might never get a "free" attack from a miss.
On the other hand, vs. a fighter6, whose second attack is at -5, you are almost assured a free attack every round or two. If you AC is not lame, rather is more like 20 by level 5 or higher, then it's almost a guaranteed "free" attack every round (from either the 1st (possibly) or 2nd (more likely)) miss.
Take it up to level 20, given a typical AC, and a fighter hacking away 4 times will almost surely miss once a round (either on the last attack or just from a bad roll).
Throw in "basic" defenses like Mirror Image, Invisiblitly, Blur, Displacement, etc. and your opponents will "miss" you a lot more (even with +100 to attack). Again, these are more likely as levels rise.
Basically, if I could take this feat as a fighter, I would every time.
All the other "free attack" feats are either rare or have problems (rapid shot -2 to hit, cleave is fairly rare, etc.). This one is like, wow. No brainer. Lemme at it.
And yes, it would make any trip specialist with a good AC just, well. Even more sick than sick. Actually almost any "special" attack would be great, for a grapler, sunderer, whatever.