An advisory board can
be 100% invaluable. I've had good advisors and bad advisors. I once had an advisor tell me to my face that "I'm a horrible businessman." He was a nice guy that I enjoyed talking with and spending time with, but he just never shared the fuller mission or had confidence in myself or my team. I've also had an advisor that has participated in the design process, rallied behind the team, and shared invaluable industry insights from his 30 year background.
The good advisors are the ones that step up to the plate when needed, providing network, review, and sometimes small amounts of work on your behalf. Bad advisors imply that they will do all of these things but somehow never do. Good advisors bring a set of experiences that can be strongly leveraged in the company design process.
It's the person, not the role, that is important.