It's massive, interesting, confusing, fascinating...and somewhat boring. It's a great idea...a city that is cold for much of the year gives students, professionals and tourists a choice of remaining underground/indoors for getting to and from destinations.
Most of it is very clean and well-kept....a few areas are less maintained. There are huge high end shipping areas (with multiple levels) food courts, lots of seating, fountains and access to the metro. Other sections are more low-key office/business/residential access.
Sometimes it's hard to get acclimated and figure out bearings....and I found the maps (and app) confusing as well.
Why boring? I'm not a big shopper...so while I enjoy the bustling areas with the nicer stores, I don't care about ever stepping foot into them. A mall is a mall to me. I like smaller, privately owned artsy-type shops....not the commercial stuff.
But the underground still a pretty neat concept and overall useful.