Feedback is necessary if you want to become a better writer. Maybe you're already good enough to get published and your first feedback will be your editor; that's fine. But if you want feedback before that, it's good to get a beta. You can't catch every inconsistency in your own.
A lot of feedback isn't helpful. As a beta reader, I expect a lot of my comments will be ignored; that's fine. It's the writer's job to decide what's helpful. Like people have noted, sometimes writer-betas get sidetracked by trying to make your manuscript into the way they'd write it. Finding betas and sorting through feedback is a long and tiring process.
The betas I've picked up have all been people I've interacted with in the forums. You can post in the beta reader forum asking for help, but that actually didn't work for me. Of the four people who looked over my last WIP, two of them were people I offered to beta read for first. One pal met me in a chat thread, noticed the quotes I keep in my signature line, became interested in the book, and offered to read for me. One person swapped with me after seeing my post in the "Willing Beta Readers" thread.
So really... if you want help, your best bet is to become part of the community. Beta reading for others helps you build your own skills.
I can't imagine using family members. Too weird. I'd use friends, but I'm a soulless people-hater. And most of my friends don't like plot-filled fantasy porn, due to reasons I can only attribute to a lack of taste.