Beta Reader? What's that?

Good post by fellow aspiring author Jenni Bailey on her blog: http://jennibailey.com/2009/10/21/beta-reading-part-one/ with a comment from me copied below in response:

"Good topic Jenni. Here I thought I knew what a beta was, which I guess I did, but funny thing is I extrapolated that an alpha would be a second set of eyes before you hit beta. Probably comes from my involvement in the game industry (where they have alpha and beta stages before going Gold - i.e. Done). Though perhaps that isn't wrong if you have someone go over stuff before you're done with the first draft.

I think your point on making sure your beta is really interested and knowledgeable in your genre is good, but I think you can also learn from those sisters, brothers, friends, etc. that aren't. It gives you a different perspective, though you have to keep in mind that some of what they give you feedback on won't really apply. This is because though your work may appeal to someone fantatical about that genre, it may be less about the quality of your work than the subject matter. Whereas if you can reach across the divide and intrigue someone not even into your genre, well that is a great thing that will hopefully lead to hefty dividends."

Any thoughts/experiences on using a beta reader? I myself am very hesitant still to use anybody outside of the "Circle of Trust". ;-)