I do Let's Plays generally, but on the occasion I've done non-LP material (my reaction to the Five Nights 3 teaser) I still go in unscripted. I find following scripts to be a touch artificial. I'll go over the points I want to make and I'm sure at some point I'll start using note-cards so I don't drift from topic, but reading from a script turns into recitation rather than discussion.
Then again, if I were doing sketches of some kind? Hell yeah, script that out.