I don't feel IF underpowered or not worth taking at all.
I'm on the fence about IF. San is right, that it is always more effective to spend points on more athletics (then wear heavier armor) than spending those points on IF. However, IF has its place;
-If you already are wearing the heaviest armor you can, due to cost or wpf constraints
-If you are already wearing full plate, then IF is the only way to increase survivability
-If you are right at the threshold between between 1-shot or 2-shot by common weapons (arbalest, bar mace, etc). Then a few points of IF effectively DOUBLES your survivability in these encounters
-It's a great place for "leftover" points; for instance, when I turn 34, I can either add 3 IF (increasing my HP by11%), or something relatively less useful like 1 str+1IF or riding