Like you, I don't mind the iris scan part. If my employer replaced my mag strip ID badge with an iris scan, that would be fine. (Great, actually; no more digging in my bag for the badge, no more phone-a-friend if I forget it at home. )

But as Ink said in post #2 above, it's the background check that's controversial. How much information is the gov't going to want in order to decide that you are "trusted"? What are they going to do with the information - how widely within gov't (or outside!) will it be available? How safely will they store it? Pistole has mentioned getting "credit card histories, tax returns..." Those might show whether I'm a good risk for a home loan, but not necessarily whether I'm a security risk. (Do terrorists pay their bills on time?)

And then as you point out, what's the benefit? Pistole has hinted at "being able to keep your shoes on" while you go through the nude-o-scope and/or get patted down. And has also hinted that there would be exceptions - on some flights/days/airports, "trusted" travelers would not get any benefit.

So - unspecified (but potentially large) amounts of personal data in exchange for unspecified (but potentially small and variable) reduction in screening. I'm not sold.

I also object because I think such a program (further) alienates foreign visitors to the US; they are often one-time or infrequent visitors who may not wish (or may not qualify) to join the program, so will face the full checkpoint experience while watching US citizens go through the short line. Not a good look for international tourism or indeed for the general perception of the US by the rest of the world.