I've watched Diablo III build videos, but if they're longer than 10 minutes, I just skip around to the parts I need. Not really the same.
I've watched one Let's Play, and that was for a Wii U game that I could not find gameplay for anywhere else, and there was no demo. It wasn't a video made by a spaz.
I'd say that they do have a place, but for the most part they don't fit for me.