Absolutely invaluable.
I used them for my first business, a tutoring business. They sent me a wonderful mentor who used to do marketing for educational companies who not only gave me marketing advice, but insider information about the education industry.
Also, if you do the SCORE program (that's what it is), you are in good shape for an SBA loan, MUCH better than if you don't do it.
And hey, it's FREE (not really -- you pay for it with taxes) but hey. It can't HURT.