Well we took the jump and entered the program. Time will tell whether it was money well spent.
The fee is $100 for 5 years. You need to decide for yourself whether the cost is worth it. The idea is that when returning to the US from overseas you get expedited through customs, though you can still be randomly chosen for questioning, baggage check, cavity search, hanging by thumbnails, etc just like any other passenger. Most people are through customs within 5 minutes, and there is no standing in line. Essentially, you are "pre-cleared" as a low-risk security threat.
To sign up, you do an online application, receive conditional approval, then go in person to your closest participating airport to be fingerprinted, photographed, and "interviewed".
I thought the interview would ask me about my patriotism, youthful indiscretions, suspicious affiliations or friendships, political beliefs, etc, but there was none of that. After all, they do a background check before the interview so they probably know what they need to already. They asked me whether I travel mostly for business or pleasure and then gave instructions on how the program works. Short, sweet, painless.
Our first chance to test this out will be early October. I'll try to remember to post our experience afterward.