Just walked through the York concourse today! It definitely is quiet initially; commuters are a bit perplexed by it -- the typical average Torontian doesn't realize what Union Revitalization involves!
This will be very different by 2023 with the extra GO RER services, fully open retail level, and commuters now familiar with the concourse. Today looks like a notched-up version of yesterday's 'soft' launch, so they can continue to debug problems with the concourse and keep plenty of space for construction crew, before putting more wayfinding up (like "New Concourse Open!" signs on the new stairwells on the platforms).
It's nice to find platforms much more easily -- it's very logically ascending as you go North-South, especially in the eastmost hallway of the York concourse, and the lines on the floor matching the tracks. It is a bit sterile and utilitarian looking without the food open yet (in a future-bustling food pit at the escalator floor) but so is other commuter stations like New York Penn Station (But ours finally looks much better now -- nyah, nyah!) and vastly preferable to the 1979 abomination on Bay street. If you're tired of the Bay nightmare crowds -- this is your concourse. For a few years until it becomes crowded.
What I think is a semi-abomination about the new concourse, is the signage:
The wayfinding is somewhat of a clutter, in my opinion. But if they have to keep the clutter, please color the empty black space between the left side / right side, as white color. That will make the left side / right side separation MUCH clearer.
But overall, this is an overall easily-fixable nitpick. Hopefully they debug the wayfinding over time. The signage appears to be mainly stickers on a solid piece of black, so fortunately should be cheap to fix (with new stickers) with improved wayfinding. I'm going to tweet to Metrolinx with that suggestion.