I don't use beta readers any more; I wonder if I ever should have. I can't believe I'm going to say this but I kind of agree with JAR on this, and wouldn't be surprised if he appeared in the thread later.
When I had betas, I would ask for brutal critiques but usually only got back "This is great!" Either I'm the second coming of Kazuo Ishiguro, or they were too scared to point out blips in the manuscript. Whichever option it was, the books were all still published more or less as
I chose to write them, so I wonder if beta readers were needed at all. The stories felt 'right' to me, and sold, so...
I've given crits to people who asked for honesty, and some have gone seven shades of apeshit afterwards. Yes, I have a reputation for being ballsy, but these crits were honest, and I gave reasons for my opinions. It seems a lot of people ask for 'brutal' when they actually want 'ego wankery'.
So I don't crit any more, except for special favours for friends.
The best crit for me, these days, is whether or not a book sells.