I can honestly say that one of the best evenings ever was having my bestest bestie in the world beta reading my novel and keeping up a steady Facebook comment stream.
First, it is instant typo repair and ferreting out a couple of small continuity issues.
"How do they know there isn't anybody left in the barn?"
"Just a second . . . "
"They do now. There isn't anybody in the barn."
Second, and even more fun, was the commentary.
"You did not just tase him."
"Shit be getting real . . ."
"Oh, just wait."
I highly recommend this to everyone who has a sense of humor about their own work and someone close to them that believes in them.