1) For the sake of FEELING secure? None. For the sake of being legitimately secure? Enough.
2) I don't think that the current system is working. Just look at how many testers have gotten through recently with bombs and/or dangerous liquids. Something clearly had to be done about this before we learn our lesson reactively instead of proactively.
3) The new security measures aren't worth anything without attentitive and healthily suspicious TSA officers to handle them. Until we cut the problem off at its base, we really aren't that secure, no matter how advanced our hardware is.