Ask HN: Your experience with startup advisors

My co-founder and I have been working on a startup for the last 6 months, and we are getting pretty close opening it up to a small group of users for the first round of feedback.

Both founders are technical and hands on, but neither has experience launching consumer web applications, and neither has the name or established following outside our immediate social circle to bootstrap a user base or a sizable following.

What we do have, however, are personal connections to some well-established entrepreneurs in this space who could be a great asset for our startup going forward, primarily in terms of introductions and advice.

What has been your experience turning a personal acquaintance into an advisor? What in your opinion would be an appropriate relationship "structure" with such an advisor, i.e. what are the typical terms? Should it be more formal or informal? I know it depends on how well you know the person, so I am just trying to get an overall sense for what I should expect and some obvious no-no's.

Looking forward to your comments. Thank you.