I've only used it twice so far for clearing Customs, and both times the GE line was quick and easy, but the regular line wasn't all that long either. So maybe I saved ten minutes each time. And that's fine and well.
But for those twenty minutes I've "saved" I spent HOURS applying for GE and going back to the airport for the interview and of course paying the fee. (Hopefully I'll use it many more times so it eventually pays off!)
I'll also mention that my husband and I applied on the same day and he got his approval the very next day, and mine took four MONTHS. I didn't even get it in time for the trip we had planned, so he got to use it and I didn't. But even then I was only a few minutes behind him.
Of course I have used it many times to go through the PreCheck line, and that is a benefit although regular PreCheck would work just as well for that and is easier/cheaper to get.
So far I have not found it to be worth it in time saved versus time and money spent, but I only leave the country maybe once a year, so someone who travels more often might reap more benefits.