Yeah, the difference when you turn it off and on is quite high too. You can definietly tell the difference
I've just completed it, and also know why you become invinsible. If you listen to a caption by pressing the action button, it knows you just want to explore and listen, so they make it so you don't get hurt.
Its not too bad. I wish they let out more details in the commentary though.
They also said at the end remember that if you like the commentry, you should report it on the e-mail he gave, since if people like the feature, it could be in future games.
Haha, yeah, I bet lots of people thought it was some expansion or something. It isn't. Half-Life 2: Aftermath is the planned expansion for Half-Life 2.