Yesterday I had the opportunity to use the below level hallway coming into the new T5 towards the customs area as I was coming home from London. I would have to say, I almost feel that while it smelled like new, the actual user experience is definitely worse than it was before, at least for now.
It was a much longer walk, definitely over 10 minutes through a very bland hallway underground. Very sparse moving walkways (one wasn't working), but the most bizarre part that from very early on, they had soft divided the walkway on Global entry left and everyone else to the right with the divided belts in between, so for almost the entire 10 minute walk they prevented people on the left from using the moving walkways, only to have the lines joined at the end right before the escalator into the customs area, so the entire divide was worthless, and the no less than 4 agents asking sporadically "global entry to the left" were essentially there for nothing.
I understand that this is just a soft opening and that the whole terminal won't be ready until 2024, but after being at Heathrow and seeing how great their airport is and how much they care about people flow even though some of the walks are even longer than 15 minutes, it was a slight disappointment.
Also very jealous of London's Heathrow express line and the brand new Elizabeth line and would love to see trains and stations like that in Chicago, one can dream
Some photos I took below