I get asked this question often. Let me tell you about my experience last Thursday at JFK.
I walk to the immigration hall and step up to the Global Entry machine. It takes my photo and says proceed to the officer.
I walk up to near the officer and and hear "Frank, come on up."
Officer: Good Evening.
Me: Hi. How are you?
Officer: Good, thanks. You're good to go.
No questions, no passport. Just go. I think the entire time it took to enter the kiosk area to the time I walked past baggage claim was about 30 seconds.
Yes, it's worth it. Especially since didn't have to do anything prior to my arrival.
Anyone else with great GE stories?