What is the value of a Coursera certificate?
Value in an academic environment: zero. Everything you learn at Coursera you'd simply learn from a textbook in a few days if you needed it for your academic research or teaching.

Value to a non-academic employer: marginal, mainly as a signaling device (this person will sacrifice his spare time over a non-trivial amount of time to improve himself), but certainly lower than a certification in a field or skill directly related to your job.

Value to a prospective spouse (as suggested by earthling): depends. I'd certainly look favorably on someone who took a Coursera course instead of watching TV.

Value to yourself: again depends, but could be highest of all. I certainly know that those courses I took were mainly for my own pleasure and benefit.