You slide on the virtual key.

Not bad... as saagarjha says, you can slide on either that key or the brightness key. Unfortunately, that doesn't work for me because I've set the touchbar to default to F-keys for the sake of consistency with the other keyboards I use. For whatever reason, this means that the slide-on-button trick does not work.

The thing is, you can't use the touch bar without looking at it whereas I was constantly blind typing function keys. I think Apple have a different understand of "Pro" than I do.

For me the physical keys are better. With the touch bar, I have to look down to change the volume, and it occasionally hangs with the volume slider visible, but not actually responding to input. That's usually when I really want to turn down the volume quickly, and instead my laptop is blasting for another second while I have to adjust the volume with the mouse.

Also I really would like a physical mute button. It's often very valuable to be able to mute in a hurry.